These heat cycles may start as early as the fourth or fifth month of a kitten's life, and will continue until she is either bred or spayed. Heat cycles in cats last from several days to two weeks or longer, and repeat every two to three weeks. You can see then, how a female cat may almost always seem to be in heat.
No one can say with any accuracy that heat cycles are painful to cats; however from the calling (loud yowling) and other symptoms they exhibit, it would appear that they are very uncomfortable.