The beautiful thing about No-Limit Hold'em is that a person can bet any amount at any given time.
The trick is finding that delicate balance between betting too much on the flop and scaring off your opponents, and betting too little and getting lots of people trying to draw on you.
On most flops when I've got a good hand, I'll usually bet half of the full pot. I almost never bet a third of the pot since I hate people drawing out on me in No-Limit.
Sure, you're giving your opponent poor pot odds with a third of the pot to call if your opponent has a flush draw or an open-ended draw. However, sometimes your opponent will hold two draws, such as a gut-shot draw and a flush draw, where he'd be correct in calling as the pot odds aren't high enough to force him out.
It's up to you to figure out how much to bet and when. There are too many situations to discuss them all, but the guidelines I've set out can help you figure out a proper strategy for the flop.
When to Make Pot-Sized Bets
The best time to make a pot-sized bet is when you feel your hand is in jeopardy from an opponent trying to draw out on you. You may even want to bet more than the pot. Since No-Limit Hold'em is about implied odds, it is better to err on the side of caution when betting too big as opposed to betting too small. Even if you're giving your opponent poor odds to call for a flush draw, you may be giving him correct implied odds.
For example, say you and your opponent both sit with $2,000. You hit the flop and bet the size of the pot which, in this example, is $100. If your opponent has the bankroll, it's correct for him to call your $100 bet because if he hits he'll usually get you to go all-in on a big pot like this. Even though the flush draw is 4-1 per card, he's getting correct implied odds to call.
Knowing your opponent comes in handy in this situation. You'll want to bet the pot size or even more if you know he'll call a lot on draws.
When to Make Two-Thirds Pot-Sized Bets
There are no set rules in terms of betting in No-Limit Hold'em. Nonetheless, most of the time, I like to bet two-thirds of the total pot size. Bets this size are best played when you suspect your opponent is on a draw and your hand seems a little vulnerable. This is so your opponent won't try to draw on you.
When to Make Half Pot-Sized Bets
Similar to the two-thirds bet, I love betting half the pot when I hit trips, two pair or any other strong hand on the flop. I also enjoy my opponents trying to draw on me by sucking them in with a lower bet so it's just cheap enough for them to keep trying for the draw. That way in the long run, they have played more money, making the pot sweeter for you.