It really depends on what games you plan on playing, but also where and how long you want to play them
If you play table games, and are on the Strip, expect table minimums of $10 and up at blackjack, craps, and roulette tables. If it's on the weekend, minimum bets go up. If you go to smaller, locals-type properties off the strip (Boyd's casinos like The Orleans, or Stations casinos, like Texas Station), you can find tables with lower minimum bets.
Even playing correct basic strategy at the blackjack tables, I'd say you'd probably need $250 a day if you wanted enough to play at those stakes.
If you play slots you can get by on a lot less. There are tons of fun new "penny" slots. They're called penny slots, but in reality you have to bet 1 cent on each of 20-45 lines to be eligible for the machine bonus. They're a lot of fun and relatively cheap to play. You could have a ton of fun for less than $100 a day.
Your money goes a lot further at locals type properties. Be sure to join the players club and use the card wherever you do choose to play. Future room and entertainment offers are linked to your play, so it pays to be a registered player.