Would 10 days Vipassana a waste of time, if not practised afterwards?

Did is did, 10 days or years is did. Karma was complete and waiting for giving resultant. Its' resultant is going to be finish in some day certainly because it's karma is finished.

Doing is doing, karma is collecting and waiting for giving resultant. Its' resultant is going on because it is doing karma, not just "did".

Both, did and doing, are good, not wasted karma, but "did" is not going to meditate or develop. Only doing is developing to get the goal.

Would a 10 days vipassana a waste of time if not practiced after?

It will never be a waste of time. In fact undertaking such a retreat is one of the most wholesome things one can do in life. I did the 10-day Goenka retreat in 2015 and it really sparked my practice. The thing is, you have to continue practicing when you get home in daily life. It's not like a button you just push when you want to. Taking breaks is not possible since the defilements in the mind also does not take breaks.

A retreat can be a way of deepening ones practice and many will experience that it solidifies their own practice in daily life even further. The external conditions found at the retreat is usually not easy to replicate in daily life which is another reason why attending a retreat is so valuable.

No matter if one does a retreat or not, meditation is not meant to be practiced only on the cushion - it should be integrated into daily life and eventually until one can be mindful 24/7.

When I went to my first Vipassana retreat I could attain to a much more subtle level of mind/jhanas than I usually was able to sitting at home, but when I went back home to practice I could no longer get jhana or even reach access concentration. I don't even practice that style of Vipassana now.

Did I find it a waste of time? Certainly not. The mind goes back to its usual frenetic state, yes, but the experience would stay with me forever, as well as the technique & insight I picked up from the Vipassana instructor. It gave me a very solid experiential backing to the theory I learnt from books. Without the experience, I don't think I could have really understood what was meant by impermanence, dukkha, arising & passing away, karma etc.

Summarised in a phrase of this generation - "If you don't use it you lose it."

In terms of this situation, 10 days of intense meditation will be beneficial for setting up a foundation for sporadic use in the future. When it comes to yourself, it would have to be in line with your cultivation goals, usage and path.

Put it into the perspective of learning to ride a bike: Takes a lot of effort at first, but once you do learn it's difficult to forget for a while. If you want to be a BMX professional it will take a lot of continued effort; if you enjoy it, continued practice becomes more of a past time; and if you want to use it for a particular agenda (in the case of vipassana, it would be application of greater insight) then conditioning your use for your purposes is the appropriate path.

Considering vipassana is a technique of introspection of one's 'cultivation'; this implies continual conditioning for the growth you are working towards (mastery of self) through means of insightful cultivation, so practice is encouraged.

When one masters a technique, are they the master of the technique? Even if we have got something out of it, is there more we can learn from or apply the technique to? Is there a higher peak of mastery we have yet to become aware of?

When we reach a point where we are not gathering more insight following one method and practice becomes stagnant, that is called a bottleneck in cultivation, like a puzzle missing a piece. When we reach a point like that, we tend to become complacent with continued practice.

To breach these bottlenecks we have yet to learn some relevant insight, when that insight is applied it results in a 'lightbulb' moment which could be considered some level of enlightenment.

Instead of using a singularly faceted approach to the technique, finding alternative methods of application encourages continued use of good time spent learning an invaluable technique.

Cultivate in harmony

It's not a waste of time.

The Buddha, prior to his enlightenment, recalled entering and abiding in the first jhana as a little boy in his father's garden, under a rose-apple tree. The memory of that, inspired him many years later.

No good practice is wasted.

From MN 36:

“I considered: ‘I recall that when my father the Sakyan was occupied, while I was sitting in the cool shade of a rose-apple tree, quite secluded from sensual pleasures, secluded from unwholesome states, I entered upon and abided in the first jhāna, which is accompanied by applied and sustained thought, with rapture and pleasure born of seclusion. Could that be the path to enlightenment?’ Then, following on that memory, came the realisation: ‘That is indeed the path to enlightenment.’