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Actually, there's not much you can tell from a star about what it's luminosity will be in the time just preceding and proceeding it finally settling down to ZAMS...w.r.t. how it generates it's energy. Actually, the star will most likely be a bit brighter in the pre-main sequence stage as it will be larger than it is when it's core ignites. Both will have some period of variability as the energy generation in the core at either stage is a bit "choppy". The only way you can tell if it's from gravitational collapse or nuclear reactions is if there is a flux of neutrinos, as already mentioned.
The time it would take to drive off it's cocoon of dust and such depends a lot on how big and bright the star is, the density of it's birth cloud etc. Each star is different, but it could take anywhere from a few decades to a few million years.
Nuclear reaction occur at varying rates within the core. Simple (first stage) PP reactions, like the ones occurring in the Sun don't take very long at all, a few milliseconds for some parts of the reaction. However the general conversion of hydrogen into helium can take sometime, several million years. Some parts of the PP reaction are quite slow.
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