What well-behaved prisoner does as often as not (4,3,4)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Double defn: 1st: If a prisoner is well-behaved, it’s possible that what he/she does/serves is half the time/half the sentence he/she was given; and 2nd: If something happens half the time, it happens as often as it does not happen.