How good does a game have to be to be profitable?

[Mempho]

I would like to hear varying opinions on this. A long time ago, Roy Cooke published in Cardplayer that, in his estimation, a game (as a general rule) needed an average pot size of 7BB to be profitable. I rarely see games that approach this number in online play (without gigantic waiting lists to get in). It is quite rare, however, to see a live mid-limit game at night or on the weekend that doesn't exceed this number.

Now, of course, we all realize that there are different ways to get to that 7 BB (short, aggressive play versus multiway, passive play). Also, there are many different ways to skin the poker cat. You could be up against a weak-tightie and hit him or her with a barrage of raises. Or you could be up against a LAG that you can 3 bet middle pair against for value.

My question is this: How do you know that just sitting down at a table is +EV to the extent that it is actually worth your time (assuming you value your time)?