The first thing to know is what is the latest you want to be on the summit and aim to be off before then. Then you calculate the estimated time it will take to summit and return. The start time is the the lastest time minus half of the trip time.
So if thunderstorms are know to hit the mountain starting at noon, and to be conservative you want to be off by 11:00 am, and the whole trip is estimated to take 14 hours, then you want to be on the trail at 4:00 am.
11 - (14/2) = 4
If getting down at 6:00 pm would mean that you get back in the dark, then it would be better to get an even earlier start.
If a thunderstorm starts moving in earlier, then you should turn around anyways.
Usually, going up and down take pretty similar amounts of time, but there are few things that make the descent faster, like
- The ability to glissade the slope you ascended
- Rappeling the route
and there are a few things that descent take longer, like
- Getting more and more tired as the day goes on.
- Loosing the sense of urgency that comes with the trying to beat the thunderstorms and so taking your time.