How much time should be devoted to an elderly parent during the course of a week?

My mother is 77 years old ,is in fairly good health. She live approx. 10 minutes away, 20 in traffic. She is able to and does use township transportation to go to the mall with her friends 3x per week. She spends 11/2 hours there and is home by 11am. I take her to most Dr. appoints which is usually 1x mo. I visit every Fri eve after work to take her shopping, banking or for prescription pick-up. On Sat eve I visit at 4;30pm, Take her to church @6pm and usually stop for coffee or ice-cream afterwards. I am getting back home @7;30 or 8pm. Sometimes I go over on Sunday any where forom 2 to 3 hours depending on where she wants to go. I work full time. Have a 17yr old that I take back and forth to work appox 3-4 times per week. My daughter is in 12th grade, I am also in the middle of college searching. My son visits my mother every Thurs eve fo dinner and usually stays for 11/2 hr. My Mothers complaint is that noone visits her enough or does anything with her. I also took her down the shore last month for 4 days. My only vacation this year. My mothers friends have large famies with lots going on. My mother is constanly telling my older son to give her a great-grandchild because all of her friends have one .My son works 2 jobs and his wife is in school and also works full-time.They are getting very tired of hearing this. He cuts her lawn every week. but this doen't count as him visiting her she says. My sister lives out of state , she visited 2 months ago and did not rent a car,so I drove her several times. She did nothing to help my mother ,. My main problem is I think I do enough but it never is enough for her.