I usually try to make a balance between the two besucase it is essential to have a goal, otherwise you don’t know which direction to go or what your objetives are. But you also have to be realistic. That will help you get real results and feel accomplished.
I dont have a set time, I work out until I’m nearly injured, and then I rest. I don’t feel bad if I don’t work out because I’m sore from before.
I prefer to set it for a realistic time because that way you are able to build a positive association with exercise. If you are always dreading it then it won’t truly become a habit. When you set realistic goals that you can achieve you will be able to have successes and any additional work towards your goals will further give your brain positive feedback!