First of all why are you bothered about play money websites. As Toby said everything applies and there are no rules but however it is suited for your best interests in real-money games. How much can you afford to gamble with. Are you a
NIT who gambles with only Aces or Kings or a loose-aggressive player.
So the buy-in depends on two factors:
- The amount you can afford to gamble with
- The type of poker player you are.
It also depends on the game - PLO, NL, Limit and the players at the table. If you smell fishes and its a PLO or NL, I would buy-in max and wait to set the trap.
There is one correction to TobyBooth: "Casino's from my experience will allow you to buy-in for whatever the largest stack at the table is at the time you take a seat"
It depends on the casino and usually in the states at the lowest level NL (1/2) they have predetermined max buy-in of 200. So, you can't buy-in for more unless your table is being broken and you are being (forced to) moved to another table and you want to play with whatever stack you are with. (Most of the Casino would accept this unless they have different rules)