Highly variable -- 50 to 100 lbs
How Much Does A Marine(or Army) Soldiers' Backpack Weigh?
When I went to Marine boot we carried about 65- 89 lbs.
The basic load is around 80-100 lbs in the Army. And me being a medic, I carry that plus about 40-50 more pounds worth of medical equipment.
They also will use a different type of backpack than what you probably use at school, very very similar to those campers use, they have a hard frame (internally so you can't see it), and a hipstrap, which changes things a lot, search ILBE in google. You can try that, or a weight vest, if you can't get those, see if you can at least find a school backpack with a hipstrap.
You could ask your recruiter (if one comes to your school), but if my memory serves, it's generally fifty pounds and up to seventy-five.