I'm not sure if you are refering to the durabilty of the pack or your ability to carry it comfortably.
1. Comfort - If you're using a Ray Way pack with no hipbelt all the weight is going to be on your shoulders. Everyone will react a bit differently but from what I have experienced and from comments here, I would guess most people probably wouldn't want to put much more than 15 pounds.
2. Durability - I can't comment on this because I've never used a Ray Way pack. My guess is your shoulders will get sore before you reach the packs limits.