When Does the Star Leave the Main Sequence?
Detailed calculations are required to answer this question, but there is a rough rule of thumb:
Most stars leave the main sequence when about 10% of their original hydrogen has been
converted to helium. Since we know how much energy is released by burning that much
hydrogen, we can get an estimate of the main sequence lifetime by comparing that with the present rate of
energy production (the luminosity).
Applying this estimate to the Sun gives about 10 billion years
for a time on the main sequence. This is close to a time of about 12 billion years coming from
detailed calculations.