There is much sensible advice here- and yes, you won't know what's hit you once you hold your first grandchild -all preconceptions will fly out of the window!
My husband did not want us to commit to regular babysitting days and reluctantly, I agreed with him, instead picking up all the emergency days (through the child's illness, days when nurseries were unexpectedly closed or when the mummy had illness etc). Whatever it was, we would drop our plans to be there and could be relied upon. Our children told us that knowing they had back-up was invaluable and took much pressure off them.
Later, at school-age, we 'pick-up' whenever needed (averaging once or twice a week for our two families of grandchildren) which entails school collections, playing, supper, bath and bedtime etc.
Holiday periods can be tricky, as the family with the child can only go away during this period of time and we feel a little bereft! Of course, we do not want to travel in peak-time, so there have been occasions when although planning a trip, we have not gone ahead with our plans as requests for baby-minding have come in and, of course... we promised....!
You will find that being with your grandchild whether once a week or more, is the most special time and when they come to you for hugs or comfort, it makes everything worthwhile. Now, our grandchildren are older but still want to share secrets or situations with us and we feel very privileged.
We are fortunate enough to be included in our families' activities and are invited to attend events, concerts, matches etc and feel we may not have this bond we have if we had not put ourselves out (a bit!) when the need arose.
Yes, I did feel a bit of guilt when friends of ours have a regular day for babysitting (on the other hand other friends have commented that we are being 'used', doing as much as we do!) but my children have assured me that our plans have worked well for them.
Do whatever feels comfortable for you and your daughter.