Essentially the answer to your OP question is infinite. You can get as many reds/greens as the universe can give on any betting streak regardless of whether it is martingale or not. This is called luck and you cannot use martingale to evade your bad luck. If there was such a method then casinos would have gone bankrupt a long time ago
Casinos don't go bankrupt for two major reasons
And none of these reasons has anything to do with the requirement of an infinite bankroll of the gambler. The first reason is that casinos limit the amount you can win (yeah, regardless of whether it is martingale or not). And the second reason is that only a small (more like tiny) fraction of gamblers are using martingale in a way that could and would ruin a casino in the blink of an eye if there were no restrictions
To keep it simple, just imagine there is only one player with a huge bankroll and a small house edge versus a huge number of small players with tiny bankrolls but still comparable to the first player's if added up. In this case, you could further reduce the showdown to just two players with equal bankrolls, one of which is using martingale against the other to overcome the latter's house edge. Who is going to bust sooner?