I think thats a very deep question and one that clearly depends on many factors. Rec player or pro player to start with - which has already been addressed in earlier posts.
For me a cash game win rate of 5 -7 BB / hr is a good win rate achieveable by a good player so if you are playing $2/$5 and winning 7BBs an hour than you are effectively making $35/hr which (depending where you live) is a decent livable wage.
Some of the best players in the world find it difficult to average more than about 7 - 10 BB / hr - this of course is over a very large sample of played hands but still, the principal is there.
If you are trying to play for a living i would say you need to look to earn between $25 - $30 / hr minimum to sustain yourself and basic living conditions.
If you are playing for fun or a hobby i think its better to set a goal and see if you can reach it
it gives you something to strive towards and play your best game to achieve.