At the end of every block? That wouldn't make much sense though (there is a big chance of loss for the pool operator if the network suddenly becomes fast).

What makes more sense: when a block is found by the pool itself (and not the whole network), all previous shares are reset to zero and the reward of that block is distributed among shares.  I'm guessing you want the latter?

(I don't think we're using that either, because it's vulnerable to pool hopping if you know when the pool found a block.)

But anyway, if you want to actually simulate this, you can no longer calculate the time when a block is found, because then it could happen that the pool finds a block without any share actually having been submitted. You now have to actually calculate the odds of one of the miners in the pool having found the block.