At triplemining, your reward is calculated by the number of shares you've contributed within a 24hr period in relation to the contribution of the other miners. For example, say you quit mining at exactly the 24 hour mark. Your reward from that point forward is shrinking as your oldest shares expire AND the other miners that continue to mine gain a larger proportion of the total volume of shares at the pool in that time frame. The pool doesn't care if you are mining at the exact time a block is found; a share's value is constant, but will expire if a block is not found within 24 hours of that share being submitted.