I think Solitary Confinement should be capped at a set time, no more than 30 minutes. To be inside a box where you can not roleplay, interact, earn money, do anything really, seems like more of an OOC punishment than something that promotes RP.
Some RP that would get you into Solitary, creates great roleplay for DOC but the punishment for the player can make it not worthwhile.
I understand actions have consequences, but this is still a game at the end of the day and if you punish too harshly then the deterrent will only limit RP opportunities rather than enhance it.
I am interested in what other people think though, as this is just my opinion on the matter.