When I was younger, I used to go weeks without changing my blades. Purely from not knowing any better, it could still do the job right with a little extra effort and going over the same area a few more times? I had no idea how bad it was for my skin. Razors are expensive little suckers after all.
I now know to change mine around once a week. I probably use them 5 or so times a week to do legs and underarms during summer. But I change them less often in winter accordingly. I do not like to become a she beast and like to be smooth for my boy but I still just can't justify shaving most days when my legs will be hidden under tights and jeans!
A tip to keep your blades sharper for longer though? Good old oil. Use a shaving oil when you use your razor and once you have cleaned your razor after use, give the blades a good dip in a jar of olive oil put aside especially for this task. You see, water causes blades to rust and blunt, while oil keeps them in good condition for longer. Just make sure you give the razor a good rinse before using again or you'll end up pretty greasy.
