When you begin to exercise you often need more rest as your body is not used to exercising so you can feel more training pain and extra tiredness. Though as you begin to exercise more you can find its harder to tire yourself out as well as not feeling as much DOMS, which can lead people to think that they didnt workout enough, But DOMS usually just happens when you do something different and shock your muscles, if you dont get it it doesnt mean that you didnt workout hard enough.
How long should a workout be...? That depends on how you are feeling at the moment and what you are doing. Somedays i just do a 30/40 minute workout where i just do deadlifts or certain exercises, other times i have lots of energy and i do intervals, 80 minutes strength training followed by headstands and stretching and the whole workout takes 2 hours or more. That doesnt mean i am overdoing it that day, it just means i have more energy. But generally speaking you dont need to do more than an hour... i would stick to 30-90 minutes, as more isnt really necessary. (But remember a 20 minute walk/20 minute yoga session/20 minute run etc etc is STILL EXERCISE AND A WORKOUT.)
I would recommend 2-3 rest days a week for best results and maximal energy each workout :) How many rest days i take per week depends on different factors.
If you find that you always want to do more and more then i would recommend that you workout with someone else. Or make a workout plan which you follow and stick to that and not do 'too much'. Instead try to listen to your body... it can be tough because we are always told that 'more is better', that you are supposed to be tired, its supposed to be hard. But sometimes its too hard and not a good hard, but an exhausted tired and hard.
A good way to know if you arent resting enough is if you feel that your progress is going backwards and you feel more tired or feel that you arent capable of as much as you could do then you know that your body needs more rest.
Below are some posts which might be helpful for you:
Feeling like your workout wasnt good enough
Beginning to workout again
Let yourself relax
Finding balance with exercise
Exercise addiction - masterpost
Listening to your body
Urge to exercise