I don't know if there is a set number (unless you are doing 8 hours a day or something) but your body will give you cues- are you so exhausted during the workout you can't go after a few minutes? Then maybe you should stop. Are you feeling pain anywhere? Then maybe you should stop, etc.
I mean if you normally don't exercise a lot but today you feel really motivated to do more- then why not? As long as you don't injure yourself it's fine IMO.
Pain, fatigue, and other body issues will let you know if you are doing too much.
Also make sure you are drinking enough water and stretching to help minimize injury
OH and ETA if you do start exercising a lot more make sure to take a day of rest now and then