Prison should last longer than 10 seconds for the mana cost. I find it hard to get much off when it is actually up due to mana and/or poor duration. It is our only hard crowd control spell, and if people are running two spitter/turrets, I think the counter should be equally as viable other than hiding in a corner or wasting mana on killing the turrets. It isn't mana efficient crowd controlling the turret 4 times, and I think counters should always be more efficient than offense. Offensively, it just prevents teleporting--you can't acid cloud or spike people in it.
If it lasted 15-20 sec, I don't think it would be overpowered considering you are using a spell slot for it, chaos leap gets out of it, and you can still block from within it (except for the shoot between the legs "feature" BUG).