Sam29, mmm, thinking about your question and trying to remember what we did. I worked off and on when ds was 2ish and MIL had him then every day for three days a week so she was very happy. I think before and after that I was just quite assertive with her about when she could and couldn't have him.
But the older he got the more I fancied the odd day or two off so she would have him then too while I got things done or did some work from home. I'm glad she wanted/wants him so much though: they have a really close and happy relationship, unlike ds's relationship with my own mum who hasn't seen nearly enough of him (too busy) and is subsequently not as close to him.
My (ex) MIL is still fantastic and I really do love her and trust her completely with ds. She's one of the few people who loves him as much as I do and I know this and love her for it. So I suppose I relaxed about it as ds got older and I got less neurotic as a parent about leaving him there etc and, looking back, I'm very glad I did.
I now live quite a way away from them but ds sees his father every other weekend so she sees ds then too. She also had him for 4 days this half term and will probably have him for a week at Christmas, 2/3 weeks (not all together, probably 2 weeks, then another) in the summer holidays and 1 week at easter, plus every other weekend.
It took me a while to accept her longing for lots of time with ds and to realise that no-one on that side of my family (by marriage) thinks badly of me - the opposite in fact - for being prepared to 'share' him (I know he's not a possession but can't think of a better word). Lots of my friends were/are envious of this close link with my MIL and of my unlimited access to fantastic, loving, family childcare if ever I want it. And I've done the right thing for all of us in letting her see him as much as is practical. See how you go once your baby is born but I bet you'll be glad of her love and support and practical help, I know I was. Hope this helps.