The owners man. says 3.75 pints. I just filled mine up this weekend and I stopped when I felt the fluid level with my pinky through the fill hole. As far as changing it out periodically, when I installed my gears, I changed the fluid out after about 500 miles or so. I didnt have any problems.
I changed my gear oil when I installed an FRPP girdle. I used Royal Purple and poured it in until it came out of the stock fill hole in the front by the drive shaft which was a little over 2 quarts. I plugged the stock fill hole and went ahead and poured the rest of the 3rd bottle in. I had problems with fluid leaking from the little vent on the top passenger side of the axle housing. I drained out about a quart before it stopped. I don't see how they get 3.75 quarts in the owners manual. I should of stopped at 2 quarts. So, to answer your question, fill it until it comes out the stock fill hole or at least close to it and stop. You should have about 2 quarts or just a little over 2 quarts. Save the rest of the 3rd bottle for your next change.