I can honestly say that I have had this problem MANY, MANY times with my boyfriend. This is also a question I've continually asked myself.
There isn't a set amount of time that a couple should spend. I think that as long as both partners are happy the way things are, then it is a healthy relationship.
Anyway, obviously you aren't happy. Now you say that you only get 3 days a week? You didn't say if they were set days but I know what worked for me, Saturdays were for quality time and Sundays were for hanging out with our own friends.
Basically, you should take some time and have a serious talk about what's bothering you. Let him know - without yelling - that you feel you deserve more quality time. He should be able to compromise.
You also said that he hasn't been in a serious relationship in quite awhile and wasn't used to the responsibilities - well girlfriend - what do you call going out for a year and a half?!?
I think he is making some excuses, and my number one rule has always been to MAKE YOURSELF HAPPY FIRST!!! Let him know how you feel. If things don't change, and it still really bothers you, I think you have to examine if this is the guy you want. Is he really worth holding on to?
On the other hand, if you choose to stay with him, and things don't get better, take those days that you aren't together and do stuff that you want to do. Like take dance classes, go jogging anything that your heart desires.
Just because he has things to do on the weekend doesn't mean that you also can't take advantage of your free time.