My husband had been cheating till he hit 60 and stopped. Then viagra (sildenafil) and active again. When will he stop?

May never stop. The honest answer to this question is "you need to ask him". It sounds like as if he will continue to cheat as long as he is able to have an erection. You need to discuss this with him.
Ask him. Some men have excessive sex drive or are even sexaholics. You must have come to terms with this in the past. It will probably continue as long as he is able to perform with viagra (sildenafil). Your problem is whether you are willing to put with it. Counselling may help you to decide what to do.