With respect to voting power:

If you vote less than 20 times per day you are not utilizing your maximum power. If you vote for 100 items per day then your average voting power will be about 1% of your voting power. Effectively, each day your voting power is spread over all of the posts you vote for. The network attempts to calculate the average number of votes-per-day your account makes. It assumes at least 20 votes per day. After 24 hours it assumes 20 votes-per-day average.

With Respect to payout time:

If no one votes for your post, then the payout clock never starts. As soon as someone votes for your post, a payout date is set for 24 hours from the time of the vote. If a second person votes, the a new payout time is calculated as the weighted average of 24 hours after the respective votes.

There is no snapshot time, everything is continuous and rotating.