Nina,
I used to keep my granddaughter every weekend too. I loved seeing her, but like you, I work fulltime, so I never had a "day off". She's 12 now, and I guess it was last year when she started having her own activities on some weekends, so she stopped coming over EVERY weekend. Although I miss her, I am secretly relieved to have a weekend off sometimes. This year, she is even busier and I wish she would come over more often!
My daughter now has a two year old, but my daughter stays home with her and she is very attached to "mom" so she's not ready to come stay with grandma just yet. I don't think I will keep her every weekend like I did the older one. I'm older now, and I just need a break now and then.