We can never repay the debt we owe our parents. So how hard should we try?

I'm up in NJ for Christmas, spending a few days with the parents. Since I work for a college, every year I have the same dilemma - how much time should I spend there? The paradox is that as the reasons for me to want to spend more time in Maryland grow, so do the reasons that I should spend more time with the parents. Several years ago, when I was living in a rented room, spending most of the break with the parents wasn't a big deal. Now, though, I own a house and a cat and have more roots in MD than when I first moved there. But now, my parents are also older and have health problems - my dad has MS, my mom has bad arthritis. They struggle to do basic tasks, and could really use the extra help. And I feel like a horrible person for not staying around to give it to them - they seem so greatful for the smallest thing. I also am the son who went away - my brother lives close by, about 20 minutes away, so he frequently comes over to do things like mow the lawn, shovel snow, ect. I don't, because I live 4 hours way, so I feel bad that I'm not doing as much.

Yesterday, I drove my mom to 7:30AM Christmas morning mass. As I stepped out of my truck, I realized that there was a bunch of black ice where I had parked - snow had melted, started to run to a drain, and froze. So I walked around and held my mom's hand as she got out of the truck. It was a reversal of roles - the child holding the parent's hand. But in the short amount of time I'm up here, I've found myself doing a lot of things that used to be done for me - cooking, driving, mopping the floors, running errands. It's weird and depressing to be doing stuff for your parents.

And I find myself conflicted about how much I should be doing - sometimes I think that since my parents sacrificed so much to raise me, I owe them the same - that I should sell my house and quit my job and move back in with them. Other times, I think that I don't really HAVE to do anything for them, so anything I do is, well, better than nothing. Reality lies somewhere between these two extremes, of course - but the question is, closer to which?

I don't have a girlfriend or wife or kids or any other obligations, so I have more time and fewer excuses not to help them out. Still, I want those things, and spending every waking free hour driving to and from NJ isn't going to help me get them (although I may be beyond help).

I'm thinking that I'll probably end up leaving NJ sometime in the next few days, which will give me basically 5 full days with them, plus 2 partial days spent traveling, and 4 days in MD to myself to relax/do stuff around the house/play with the cat/run errands/hang out with friends. I'm not sure if this is the "right" amount - if I'm a sinner, a saint, or just a typical son.