This is an issue that is ripe for abuse by Police.
I've given chav scumbag RP to Police before and it rightfully annoys them, they then give abusively long sentences (absolute maximum) for the crimes that you've done, even if within RP you provide multiple mitigating factors.
I've always done 100% of my prison time in-game, beacuse I can cause trouble on tweedle, chat with mates etc. But for people who are more casual (or new) on the server, serving 12-hour sentences because the cop that was locking you up was having a bad day, or was a wanker in general, will just mean people won't come back, because they're forced to spend hours of their life on the server sat in a small area.
This suggestion also heavily favours big gangs who can organise breakouts, whereas the newbies, or lone wolf style players will again be shafted, and if they're lucky enough to get out of prison in a prison break, they're still buggered because the big gang isn't going to let them out.
This issue would be far better with a total rework of the prison system, with an independent person from the Police deciding sentences based on actual legal principles. Whether that involves hiring more judges, I'm not sure.