Nothingness:

If you set up a new game with bonus time the default setting is 7 days to start with a 1 day bonus per move and a 15 day maximum for time allowed.

I think that's a very reasonable time control because:

1. Overall game time is roughly 1/3 to 1/2 of the normal 3 day per move limit.  Longer games, those that cause scheduling headaches, will be close to 1/3.  An 80 move game would have a max length of 174 days compared to 480 days using a 3 day limit.

2. Players needing time-off can usually "bank it" if they plan ahead and avoid time-outs.

3.  Anything shorter than 24 hours for the bonus time runs into possible timing issues.  E.g. Player A logs in 8pm ET each day and moves and Player B logs in 9pm ET each day and moves.  Player A will lose time every move against Player B and often be at risk of timing out even though they both have comparable schedules.