how is the orderfiller incentive calculated?

Rate is based on your building's CPH (cases per hour) target (I don't even know my own building's target right now.) Once you start pulling over 100% of that, you start getting paid. Whatever percentage over 100% you pull is how your incentive will be calculated. Remember, this only applies in grocery DCs. Regional DCs don't have any sort of individual incentive that I'm aware of.

how realistic is it to do all the time?

If you can get into a groove and learn how to economize your movements, very realistic. You'll be with a trainer for your first 40 hours, and they should teach you everything you need to know.

How regularly is overtime offered?

There will usually be a sign-up sheet around the major "food" holidays, and you can also sign up to receive a text whenever your building needs help. I've also heard of some guys at my building calling a specific desk on their days off and asking if they need help.