I guess I don't really know what "time for a relationship" means. My marriage is pretty much integrated with the rest of my life... I don't think, "Oh man, I am spending WAY too much time on this relationship! I need to cut back!"
It doesn't work that way, at least not for us, especially when we live together and don't have kids (since having kids takes away time for each other, though you're still working as a team to raise them).
Ever since my husband and I finally started cohabiting, we are pretty much together whenever we are not working. So that's in the morning before work, and in the evenings on weekdays (not including the twice-weekly excursion for his language classes, or our individual time surfing the internet in the evening... so subtract a couple of hours a day for that), and pretty much all but a couple of hours (sometimes) on the weekends. So I'd say at least 3-5 hours each weekday, and... 12 hours a day on weekends?
We do nearly all of our errands together (we only have one car, after all) and since we are living abroad and don't have many friends here, we don't have a lot of external socializing going on. That may change once we get back to the US, or at least I hope it will, but I still can't imagine that taking more than a couple of hours every week. We aren't far apart from each other when we're cleaning, cooking, doing other chores... and those don't really take long when you are doing them as a team. Apart from our circumstantial factors, we just REALLY love being around each other, even if we're just sitting next to each other while using our respective laptops... especially after two years of long-distance. That makes us appreciate all the moments we have together, I think.
There are many days when we go to bed at night and I think, "Man, I just can't get enough of this person," because I really can't. There's always more I want to talk about, laugh about, hear about, snuggle, have sex, basically love each other... and there just aren't enough hours in the day to get that all done. But I think it's a good thing to always have a little longing for that significant person in your life, just so that you don't take them for granted, and so that there's always something to look forward to doing with them.