Kurtosis,
It usually takes about 13 to 20 weeks for a femur fracture to unite. But, this can be less or more, depending upon the age and overall health of the patient. The usual way to know for sure when a femur fracture has united, is to x-ray it.
If the surgeon has allowed you to go to a cane, that actually means that you are putting 50% weight across the fracture (that's the max amount of weight reduction that a cane can provide). But, if the surgeon has told you only 25% and thinks that you are still on crutches (ie you went to the cane yourself), then you need to be very careful.
In the fixing of fractures, it is always a race between getting the bone to heal before the hardware fails. There is no hardware made that can withstand continued stressing. Unless the bone heals, all hardware will eventually break. Sort of like a paperclip being bent over and over, it will final snap.
Even though you do not have pain at the fracture site, that does not mean the fracture has healed. It is a good sign, but the IM nail and the early callus may be holding the fracture fragments together. But, this is not stout enough to take full weight bearing.
I understand you want to get going and get over this injury. But, try to follow your surgeon's instructions. He/she knows how strong the fixation is and how healed the bone is. Some weight bearing is good, because is does stimulate the body to lay down more callus. Bone will respond to the stresses applied to it, called Wolff's Law. That is why bone will become stouter and stronger when you lift weights, and why it becomes osteoporotic with disuse (age, injury, space flight).
Unfortunately, knee pain is extremely common after retrograde IM nailing. Anytime the knee joint is violated, there is a possibility of inflammation and scarring within the joint cavity.
Being only 8 weeks out from surgery, you are still very much in the healing phase. Before going to full weight bearing, be sure the bone is strong enough to withstand the stress. Or you can break the nail, putting you back to square one.
continue to work on active knee flexion. It is very common for patients to lose knee flexion after a femur fracture, because of the damage to the quad muscles. The quads can scar down, thus limiting knee flexion.
Good luck. It sounds like you are doing very well, but don't push it too far.