Probably 2.5 years ago when I weighed 55 pounds more... I tried my best to exercise at least 4 days a week even then, but my knees were so bad that they, they being everyone, doctors, family, and friends, wanted me to have bi-lateral knee replacement. I was questioned at this time at work about being able to preform my job duties as I literally could have been in a wheel chair; that is how hard it was for me to move around.
"How long to significantly increase muscle mass?"
Not long at all. If I lift, I gain mass... do not know why, I have always been this way. I can also lose it if I move towards a more cardio emphasis, which I may do IF I can get back into running over the coming years.
"Looking for some idea what to expect as I stay active and do weights regularly now."
Walk, bike, elliptical, mini-tramp, and of course any housework/garden work.