This would depend on the distribution of times at which other people are minting and redeeming Zerocoins ... but under reasonable assumptions, you'd probably want to go with something like the exponential(1/N) distribution, since that's the maximum entropy distribution on [0,∞] having mean N (http://en.wikipedia.org/wiki/Maximum_entropy_probability_dis...). This has the somewhat surprising property that the most likely time for you to redeem a Zerocoin is immediately after minting it!