http://www.afge1213.com/roulette/roulette-when-it-is-the-right-time-to-bet-on-numero-zero
Roulette, like other casino games, is a game of chance and hope. It is about choosing the right number, and willing that little metal ball to hop into that pocket. Like any great game, there is always the perfect time to make every move. In the game of roulette, it is up to the player to decide when that move is to let it all ride on zero.
When considering when it is a good time to put your money on zero, the shortest answer is, anytime you are playing with a European table, rather than an American table. A European roulette wheel (the style of which can be found in many American casinos) does not have double zero, it only has zero. This improves the odds of winning with zero from 37 to 1 to 36 to 1, while the payout stays the same. That may not sound like a lot, but it means that in the course of an hour’s worth of play, someone is likely to hit it with the single zero.
Playing roulette is about trying to play the odds, and guess what will happen next. Many players do this by simply choosing a number that feels right for them. It happens – they “feel” a zero, play it, and win. But more often, successful players have a plan of action.
One method is to watch the winning number for quite some time. Once the ball has chosen a red or black pocket for perhaps 20 consecutive spins, a zero can start to feel pretty good. If you play the odds, there will be a zero in the next 16 to 17 spins. Thus, if you were to put a five chip down on the zero spin after spin (starting at the 20th red or black spin), even if it took the full 17 spins, that would cost you 85 chips, but if the zero came up on the last spin, it would pay 185 chips, for a net win of 100 chips.
If zero feels like a good number, but you do not always trust your instincts, then one way to hedge your bet is to box the zero, playing zero, 1, and 2, or even zero, double zero, 1, and 2. For many people, the green zero just feels lucky, and in these cases, any time is the right time to put it down on zero.