Usually I am of 2 camps:
When I am feeling "on-top of things" I thin pretty early, just choosing the biggest strongest, and either repotting the thinned plants to gift, or do experiments with. say 3ish weeks?
When i am feeling lazy, I let them go for FAR too long. I think I currently still have some "extra" tomatos that are sitting in a 6-cell with their twins still alive (though stunted). They're at 6 weeks now.
When I first started gardening I was super diligent, and have relaxed as time has gone on. Through curiosity, I have often let things go in less-than-optimal conditions just to see how they fare.