I think you need to be pro-active about these things, especially when she's nearby and you are committed to a harmonious relationship in the long term.
Next time she visits, how about an adult conversation about it:
"MIL, now that we have baby, I know you'll want to visit often. I'm so glad you are keen to be in baby's life. I'm not really a dropping round randomly or unannounced kind of person so let's agree a regular time(s) that suit all of us so that there's no awkwardness going forward in the long term"
Then suggest start and finish times and frequencies that suit you. E.g. 11-2, 3-5 on a Sunday or whatever. Keep using language that leaves no vagueness -"so 2 hours on a Sunday morning then?" until it's agreed.
Absolutely confirm these as a date and tell her you will put it on your calender.
The first few times have something in mind you are doing at the end of the allotted time (e.g. "it's been so nice to see you, DH and I are off to do xyz...got a film planned for...would you like a cup of tea before you go...").
It takes a bit of training, but you just have to master being polite and assertive. You aren't responsible for her emotional baggage or problems respecting normal boundaries.
She may balk, complain it's too formal etc -don't respond to any criticism of the plan as though it's personal -a vague "I guess we're all different" "Different people have different preferences with this sort of thing" "Best to avoid any bad feeling by agreeing something that suits us all" "Would hate to have resentment build by not agreeing something that makes us all happy".
Life changes, and dealing with it like this allows you to make explicit adjustments in the future. "MIL we have a stressful and busy week with xyz going on, can we drop the visit on a Wednesday and we'll see you for your usual Sunday visit."