I am planning to make a bitcoin betting site.
...
You'd need a serious marketing funds. The competition is pretty tough and people don't trust the new sites. You'll have to earn the trust, and will have to offer some incentives to attract the players. You may be running at loss for the first few months/year.
As for life span - if you get everything right from the start, prove you're trustworthy etc you can last for decades (unless bitcoin dies).
How big do you think is that marketing funds? $5k, $10k? If that is not enough, what is enough?
I have no idea on that, but you can get some idea from how much other gambling sites are spending on signature campaign.
Example 1 LB: 20x 0.1 btc slots + 20x 0.05 btc slots = 3 btc a month
Example 2 BitDice: The owner said his budget for the first month is ~2.5BTC
Example 3 Pocket Dice: 50 slots, 0.001+ BTC per post (depending on member rank), max 25 posts per week