I think market demand should take care of this naturally. If a freeroll has a low prize pool and a long duration, well that's not very attractive so not many players should bother entering, so those that do enter should have a better chance of winning which compensates them for the time invested. If you made it a shorter duration then it's more attractive as there's less time invested, but then that attracts more players so there's a lower chance of winning making it less attractive, and so on
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