What is the length of time for each fusion process in a massive star?
My stellar astrophysics text, Francis LeBlanc's An Introduction to Stellar Astrophysics, gives the following quantities for the stages of burning in a $25M_{\odot}$ star (citing models by Arnould &amp; Samyn 2001). This is somewhat less massive than a typical main sequence O5-type star, and with higher masses come higher core temperatures and shorter evolutionary timescales. Nonetheless, it should give you a reasonable order-of-magnitude idea of how long it takes for these processes to play out in a massive star.




Phase
Central temperature (K)
Timescale




Hydrogen
$6.0\times10^7$
$7\times10^6$ years


Helium
$2.0\times10^8$
$5\times10^5$ years


Carbon
$9.0\times10^8$
$600$ years


Neon
$1.7\times10^9$
$0.5$ years


Oxygen
$2.4\times10^9$
6 days


Silicon
$4.0\times10^9$
1 days




(One major caveat is that these models are two decades old, and stellar evolutionary codes are constantly being improved. However, I don't believe that these are old enough to be significantly out-of-date!)