Since you're doing a partial boil, you have a lot of leeway. Whatever you end up with at the end, you're going to make up with water.
That being said, too small of a boil volume (some would say anything less than a full batch boil) can impact the flavor of the beer in ways you may not want - a longer boil will accentuate this.
I recommend figuring out the boil-off rate of your boil kettle, and working backwards from there to figure out how much water you should have to wind up with X gallons at the end. There are calculators on the internet (or Beersmith) to help with this.
2.5 gallons is probably a little on the light side, depending on your boil kettle size. With an almost 2 hour boil, you would likely lose at least half of that.