I believe I get mental burnout more from not riding. There are days that I just do not feel like riding, but I do not think it comes from riding too much. It comes from having other things to catch up on, lack of quality sleep, or maybe just a bit of depression. I also realize that for me, having some kind of goal helps to keep me motivated to get on my bike regardless of weather or other non-riding issues/events. I always perk up if I just get out and start pedaling.