In NL, my win % at showdown is 75% or more during a really good session. When you take a hand all the way to the river you better show a solid hand. However, like Fnord mentioned, NL is not a "showdown" game ie. you should win a lot of your pots without showing your hand.

In limit I think anything over 50% should be profitable, if you win more than half the hands you take to the river. Those numbers are always decieving because some pots are AI or capped and others are blind steals. Don't let those numbers dictate your play too much.