No one can give you a solid answer here. It is different for all of us. I can play 6 hours of 12+ cash games and be just fine. Some may have difficulties with 4 tables for 2 hours. It's more with what your comfortable with. Start slow. If you do decide to multi-table, start with one and add one as you go. You may go too far, that's okay. Just tone it down to where you are comfortable.
As for designated time, that depends. Is it a hobby? If so, then it doesn't matter. Is it for trying to learn to play the game? Maybe twenty minutes studying, two hours playing. probably not that vast of a difference, though. If you're trying to play serious, Having a weekly hours played is a good goal in my opinion, but you shouldn't force it. That's what I love about poker, you must take it as it comes and let it be if it doesn't. Long ago, I could play 10+hrs and still be confident and making good decisions. Today, i lost two buyins and decided, to hell with it. I ended my session early. Although i was hoping to add on a few more hours, I just didn't feel right playing.
So to answer your question, it really depends on what you're trying. I would strongly suggest studying(videos, books, review) to improve your game considering you're investing time and (most liekly) money into it, so you'll wanna make sure you know what you're doing. In terms of playing, start small. Very small. Don't try to advance in time, stake, or number of tables until you not only beat the level, but also feel confident in yourself to push yourself further. Best of luck to you on the tables!