I'd say at least a week for a felony. For instance, if someone gets booked in for Murder on a LEO that person should stay in prison for at least a week, to be honest. It's totally unrealistic if that person goes out of prison the same day or the next day and goes back to roleplay with his gang. That's a no-no.
Agree with Bryar on this one, give out 2-3 day sentences but the time will go down OOCly, doing this will probably discourage people from doing crimes so possibly opening new horizons in legal RP for some people. I think this would be great as then when you go to jail you won't have people AFK everywhere etc. Also on the topic of prisons, I think more roleplay opportunities should be offered, for example:
Being able to work in the prison yard for some extra cash
A realistic way of getting you hands on weapons which also requires dedication in your RP (Not like that bs we saw on LSRP where u take a shaving razor and toothbrush and rping squishing them together and getting a knife)
Offer rehab
Offer an in prison school for younger offenders
Showers :) In all seriousness though showers can provide some amazing RP (no not rape)
And many more along those lines. Now it is arguable that these things can all be done ICly but I think with some OOC help they can actually become a reality and be effective in the long term. I'm just hoping that the admin team is ready to experiment certain aspects and not be afraid of trying new things.