How to do metta for a long time (before sleep)?

One may start to face him/herself and - by remembering/reflecting on ones done good deeds and qualities - thinking - wishing "May I be at ease, my I gain peace and liberation with ease", then, by dedicating at the same times ones done merits, swift to the most beloved, best most obligated (teacher in Dhamma, admirable friend, Sangha, parents...), best by name or imagening, "May X be at ease, may he/she gain peace and liberation, by him/herself, with ease", then further, friends, all you know who come to mind, best one by one, the "bad one", next village, next countr_, the seen, unseen, big, small, every creature, what ever lives, ... in all directions, all realms... it's good to verbalice and imagen as long as possible, aloud or by thought. If not having falled asleep till got all cover all, without doubt, you probably will not care much about sleep/not sleep, prefer to dwell mindful and watchful in this realm.

"For one whose awareness-release through good will is cultivated, developed, pursued, handed the reins and taken as a basis, given a grounding, steadied, consolidated, and well-undertaken, eleven benefits can be expected. Which eleven?

"One sleeps easily, wakes easily, dreams no evil dreams. One is dear to human beings, dear to non-human beings. The devas protect one. Neither fire, poison, nor weapons can touch one. One's mind gains concentration quickly. One's complexion is bright. One dies unconfused and — if penetrating no higher — is headed for the Brahma worlds.

"These are the eleven benefits that can be expected for one whose awareness-release through good will is cultivated, developed, pursued, handed the reins and taken as a basis, given a grounding, steadied, consolidated, and well-undertaken."

Metta (Mettanisamsa) Sutta: Good Will

Good detail teachings on Metta, also in relation to Vipassana, one may find in Ven. Nyanadassanas Words on Karaṇīya Metta-Sutta

A probably good way and of course most traditional, would be if putting effort into learning to maybe recitate the Karaniya Metta Sutta You may make use and follow the Venerables of Wat Metta:

Audio download (pali/engl) and the text to it. Then chanting it, while starting to follow above steps.

Having later found good understanding of metta, one might arrange ones "own" gift with short or extended stanzas.

Metta & Mudita

[Note: This is a gift of Dhamma and not meant for commercial purpose or other low wordily gains by means of trade and exchange.]

Vipassana meditation and Tonglen practice are from the Theravadin tradition and the Mahayana tradition, respectively. Within their own traditions, they have very different objectives. Vipassana meditation has the objective of uncovering psychological insight into one’s own sankhara, while Tonglen practice has the objective of overcoming the self-centeredness of the belief in self (that must eventually be overcome by the experience of sunyata). Since your teacher recommends the practice of both (vipassana followed by Tonglen practice), I would recommend (only after you consult with your teacher) that you put enough time (perhaps a half hour) to allow you mind to “change gears” from an intensely dry and objective meditation to a deeply emotional practice.

how do i extend the metta time ? - just repeat the metta cycle again and again and again ?

Consistent and diligent practice together with the balancing of the Five Spiritual Faculties, especially; Energy & Concentration.

Each week one could try to increase sitting with e.g. 5 minutes.

You can try Tonglen, it's a kind of Metta meditation when you serve as the filter for the world. Breath in all trouble, pain, and confusion of the world and breath out peace and happiness. You can do it for a long time until you fall asleep.

just repat the metta cycle again and again and again?

I guess you mean "repeat the words"?

I think not.

As an analogy: if I decide to go somewhere, then I might be verbal or have some specific direction[s] initially -- I might decide, for example, "I will travel, to that place". After I have started, while I'm going there, however, I won't keep repeating that same phrase -- instead, I am conscious of 1) being somewhere 2) moving in a direction toward a destination 3) what I see on the way.

If I decide to start meditating, instead of what I was doing before, then discursive thinking (producing words) stops. Instead of thinking about words, I think about "real" (defined as "actually existing as a thing", so physical as well as sentient) people, who as people exist here and there in directions more or less far away from me.

Thinking about that physicality (with the intention of being kind "in real life") switches off my discourse. Switching off my discourse, my breathing changes, instead of shallowly I breathe deeply. Conscious of suddenly breathing relatively deeply I'm conscious of being more relaxed, of well-being, of well-being being associated with metta.

Conscious of well-being (or of "travelling in the right direction") I have no desire to change that direction, and instead want to continue in that state. If you're travelling (for example on foot or by car) and start to veer towards the right or the left, then you correct the steering to resume the right path/direction. If I start to leave the state of well-being then I'm conscious of what I'm beginning to do wrong (for example, thinking about myself instead of expanding my conscious attention to include and wish well towards other people, or for example beginning to think discursively again), and undo that (don't continue in the wrong or less rewarding direction) to return to the right direction.

Doing this in bed is especially pleasant: relaxed, warm (under cover), eyes closed. Eyes closed makes it easier to expand/extend my awareness (or imagination or consciousness) of space (instead of being stuck behind my eyes and looking in one direction, i.e. visual consciousness).

Warm and relaxed, and breathing, contributes to the feeling of well-being. Sometimes my thought (or focus of awareness) comes back to my body (instead of being 'out there'). I think that's because it "needs attention" e.g. it's tense or uncomfortable in some way ... it's enough to consciously relax that part (musculature) that feels of tense, for the feeling (of tenseness) to go away, after which my mind is free to leave again.

It usually isn't very long before I'm asleep, though. See also Meditating as sleep aid.

Another pleasant thing about a metta meditation is that it's time to do nothing except goodwill (i.e. wishing people well). It's time to stop making practical plans, to switch off the discursive machinery.