How much tummy time do you give your babies?

DD2 is 4 weeks old now & HATES being on her tummy. She gets plenty of nappy free time & loves kicking around on her back but only lasts a few minutes on her tummy before she is grizzling. She will sometimes settle off to sleep lying on our chest on her tummy but I don't think this is quite the same.
Not sure how much tummy time to give her... I feel mean doing it to her when she obviously doesn't like it, but I know it's good for them. I never had this with any of the others, they all loved being on their tummy.
Any ideas?