I think you are quite reasonable, hell you don't even have to accept overnight visitors... ever! Afterall, you decide what you are comfortable with and whoever wants to visit can either accept booking a hotel or just not seeing you very often. No one gets to bully you into hosting them!
We'll be having another little one in December, due on my DSS (dear stepson)'s birthday, which is just after DH's birthday and right before mine (not to mention Christmas) and I don't feel like having visitors to entertain when I get back home with a tiny newborn to a 3yo DD and 7yo DSS who will also have to adjust to life with a new sibling. So if my in-laws want to visit they can come after a week or two (for Christmas) and entertain themselves! Though I doubt this will be a problem for us since MIL usually takes over all my chores when she's around (it's alright with me, she always asks) and if she's tired DH knows that he'll be doing the entertaining.
If your in-laws are the kind to sit around and expect you to wait on them right after you've given birth then I say you shouldn't set a definite amount of time before you'll accept overnight visitors. You don't know when you'll be feeling up for it and even if your DH takes over all hosting duties it's going to drive you up the wall not to be able to host them as you wished. Stress and milk supply don't mix.
If all else fails and your in-laws make a big fuss, just put on your bravest face and go:
You have to have a very sincere laugh followed by a deapan and firm No, it'll confuse them into being speechless. I'm not always a very polite person when my boundaries and comfort are disregarded...