2.5 hours is a lot. I think that there are many people who can't even deal with that.
Break it down.
I have learned during a self-help program developed by an expert in learning that learning 30 minutes and then taking a 5-minutes break, learning 30 minutes, 5 minutes break, learning 30 minutes, 20 minutes break would be the pattern our brain would take best. During those breaks you have to stretch, drink some water, leave the room you're learning in.
Also, how are you learning? Have you tried alternative ways of learning? Mindmapping? Photoreading? If you prepare now for an exam it is not a good time to learn something as complex as Photoreading, but mind mapping is extremely useful because it helps you structure the information and uses both brain hemispheres, which help long-term retention. If you want to try it, here is a free tool for mind mapping.
If you can, do your best to involve best brain hemispheres by using drawings, colors, music, anything that involves an image, a sound, a metaphor that can be associated with the information you try to take in. It takes away the boredom and increases learning efficiency.