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Hi Suzy;
Bear in mind, there's a lot of variability out there.
Also, They still haven't figured it all out, yet. We tend to forget that the technology used to observe all the stars necessary to develop credible models, has really only recently become available. When I started reading up on this, I thought all this was already known. I think the difficulty you experienced in attempting to find a straight-forward answer, indicates that the story about star evolution is still in its infancy.
I mean - look at Ron's 'Trillions of Earths' thread, and there was another .. (forgotten what it was called) .. which showed that there are a lot more stars, (& exoplanets), out there than was previously thought. If there's a lot more stars, then the conditions of their formation may be more varied than we think, also.
I guess, theoretically, fusion starts in an instant, once you have the right conditions for it … the question is really how long does it take for the conditions to occur, for stellar fusion. (Same thinking would also seem to apply for proto-planet formation)
Cheers
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