How much time, is in fact, too much?

Honestly, think about it, people say when you have a boyfriend or girlfriend that spending a lot of time together is a bad thing. So, why is it that when you are married or living with your significant other it's not a bad thing? I am thinking that if you are able to live with someone knowing all their flaws and weaknesses that it should be a good thing. On the other hand even if you're able to put up with someone that much, too much time toogether can cause arguments. Whether you're married, living with someone, or just dating them, it's always a good thing to spend some time apart!