Seeing as BESM does not (to my knowledge?) have any rules for extended rolls, here’s the ruling in case it should become necessary.
Not every task can be solved instantly, with a single effort. Occasionally, there might be a reason to break a single task down into smaller steps – painting a picture, building a machine, or other activities that may take several days to finish. Certain other activities might also be more exciting as a drawn-out challenge rather than solved with a single opposed roll, for example, a football match.
When the rolls are against a static difficulty, first determine the basic difficulty to progress with the work. By default (just like with all other rolls), this difficulty is 12. As with all other rolls, the difficulty to progress can vary from trivial (6) to impossible (24) or even higher. The difficulty to finish the project can vary as well.
For every 3 points by which you beat the difficulty on each progress roll, you get a +1 bonus to the roll made to finish the project. These bonuses are cumulative – more preparation means a higher chance of succeeding in the final roll. A very skilled person might be able to skip directly to the finishing roll, representing the ability to work incredibly quickly; most people will need 2-3 rolls and an unskilled person might need to make five or six.
If you do not beat the difficulty of any of the preparation rolls, either nothing happens (you make no progress towards the finished project), or you fail the whole project and must start over from scratch, as appropriate.
To finish the project, make a roll as usual with all the bonuses from your preparation rolls factored in. You may not spend Energy Points on this roll. It represents the collected sum of your effort, not a single task you can pour your heart and soul into. You may spend Energy Points as normal on the preparation rolls, as these represent concrete tasks.
Sometimes there simply is no difficulty – your roll result just represents the quality of your project. Other times, you must beat a certain minimum difficulty, or the project fails. As always in Big Eyes Small Mouth, use common sense.