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I would view the answer to this through the lens of right effort. Your practice should aim to be formal enough so that you stick to it and fluid enough that you don't just end up going through the motions. That said I would err on the side of regular times and being a bit stricter with yourself - particularly if one is just starting out (not certain that applies to you).
As far as hindrances go I wouldn't let that stop from either meditation practice. I've been advised to use the hindrances as an object for meditation so could potentially led to some interesting insight.
More generally, at various points I have been give various advice to help maintain an effective practice. So in no particular order
Find a particular time of day that is better for you to practice. Typically this would be early mornings or late evenings but doesn't have to be
Get a place for meditation. Set up a shrine if that helps but have a spot. Keep your cushions set up so you can just slide onto them when the time for meditation is there
It's better to meditate for 5 minutes than not at all. Sometimes you can just sit and connect with yourself and others as best you can. It's enough.
If you can't meditate one day don't beat yourself up about it. Be very kind and patient with yourself. It's a long journey.
Generally it's the middle way which seems easier said than done. But that's the middle way for you.
Upvote:1
According to the Maha-Assapura Sutta a practitioner abandons the hindrances before working on insight.
Practice samatha meditation until the five hindrances dissipate.
Abandoning covetousness with regard to the world, he dwells with an awareness devoid of covetousness....
Abandoning ill will and anger, he dwells with an awareness devoid of ill will...
Abandoning sloth and drowsiness, he dwells with an awareness devoid of sloth and drowsiness, mindful, alert, percipient of light....
Abandoning restlessness and anxiety, he dwells undisturbed, his mind inwardly stilled....
Abandoning uncertainty, he dwells having crossed over uncertainty, with no perplexity with regard to skillful mental qualities....
Seeing that they [the five hindrances] have been abandoned within him, he becomes glad. Glad, he becomes enraptured. Enraptured, his body grows tranquil. His body tranquil, he is sensitive to pleasure. Feeling pleasure, his mind becomes concentrated.
After abandoning the five hindrances work on vipassana. Some days you may use your entire meditation working on calming and removing the five hindrances through single point meditation.
With his mind thus concentrated, purified, and bright, unblemished, free from defects, pliant, malleable, steady, and attained to imperturbability, the monk directs and inclines it to the knowledge of the ending of the mental fermentations.
He discerns, as it has come to be, that 'This is stress... This is the origination of stress... This is the cessation of stress... This is the way leading to the cessation of stress...
These are mental fermentations... This is the origination of fermentations... This is the cessation of fermentations... This is the way leading to the cessation of fermentations.
It is important to work both samatha and vipassana rather than skipping samatha and waiting for days when vipassana is easier and feels more natural.
Upvote:1
In my opinion, vipassana should not be forced at all.
Forcing oneself belongs to samatha, targeted at developing pliancy of mind. However, according to instructions I received, even samatha should not be forced as much as to develop antipathy towards meditation.
Luckily, vipassana (unlike samatha) is a kind of meditation that can be done "in action", without sitting. So even resisting meditation can be turned into meditation, by paying close attention to the experience of resistance.
Upvote:1
I personally have found that if I don't schedule sitting meditation into my busy busy life, I fail to do it as often as I should. What's more, I have to schedule it daily in the morning to be certain I won't be weak and procrastinate until it's too late.
I also practice meditation any time I have to wait for something. For a delightful introduction to momentary meditation see How to Meditate in a Moment.