You generally don't develop a callous on the lip but you do develop a better resistence, I guess the lip does seem a little tougher in that area but it's not like the type of callous you get on your hands or feet. You may develop a blister. If you have braces or bite too hard this may make the situation worse. Playing long tones really helps develop the muscles of the embouchere which in turn relieves some of the pressure on the lip. In order to really develop a stronger lower lip you need to put in hours of playing a day every day. Pay attention to your body, it's like exercising. You have to build up don't just start by playing 8 hours one day and expect it to be better.