You can meditate as much as you can handle, it's an exercise in a sense so if you start getting headaches then you are overtraining. It can be different symptoms which are similar to exercise overreaching and overtraining like irritation, mood swings, feeling tired, etc there are too many symptoms.
I know a symptom I get when I overdo it is that my mind just feels tired and can't visualize the same way or it sorta hurts my nervous system.