You only need one promotion, L3 (entry level) to L4 (mid level), to not get fired at Google. And the company is quite generous with how long it gives you to get there. L3 to L4 is basically about writing good code on your own, and isn't really affected by promo culture -- doesn't really need shiny work.

Several years back, you had to also eventually get to L5. This was more affected by promo culture, but was also largely unenforced, which is why they get rid of the formal requirement to get to L5.