No, there aren't a minimums.

The time limit is two years from the end of your accounting period. Tax credits are a Corporation Tax relief and the deadline for amending your Corporation Tax return is usually 24 months after the end of your accounting period.

Not at all. In fact, if you attempted an R&D project and it’s in different stages that’s a good sign that you’re doing qualifying work. Moreover, if you’ve worked on a project and it was unsuccessful, that is no bad thing. If you’re unsuccessful that showcases that you are attempting a project with an uncertain outcome, which is exactly what HMRC want to incentivise and reward.