2.5 hours should be pretty "easy". We've done several trips, mostly around 6 hours each. Make sure you bring a few favorite toys and of course food. If your baby takes a bottle, feed her on the way up and down, or use a passy. Our baby recently rejects the bottle and passy, so we tried to bring a lot of chew toys and that seemed to work.
One thing to consider is the flight time. We tried to time it so he would be starting his nap as we were taking off. The hum of the plan and light movement had him sleeping for quite a while! Southwest is great since there are so many kids on those flights anyway
