Hmmm, there's a lot of variables in a question like that...
The life of your battery will depend on things like how new your battery is, the temperature (they don't perform well in the cold), etc.
How many spare batteries do you have, Vincent?
If you have spare batteries, and are concerned about the life of them, then I would suggest taking shorter subs and stacking them. That way, when the battery low warning light comes on, you'll stand a chance of finishing that exposure, and then quickly changing the battery to take the next sub... You might ind a slightly longer gap in the star trail when you change the battery though