We lived with my in-laws when our son was born because we had just moved cross country, waiting for a house to sell, etc. It was already pretty stressful and though I love my MIL, she doesn't completely get me and isn't the a natural nurturer. My mom lives overseas and had planned to come later like 2-3 months after since she assumed I had all the help I needed with live in grandparents. But, 1 week after baby, I called my mom in tears and said, Come, NOW! She came and stayed for 7 weeks! I was really sick pp and you can imagine it was crazy stressful with my mom, dh, me, baby, in-laws all in one house! It was like war of the grandmas!
Anyway, it was wonderful, my mom anticipated my every need, took out trash, fixed meals, shopped, made sure I got up and exercised, took baby at night so I could sleep...I mean, it was wonderful! I have already told her that when and if baby #2 comes, I want her here (now in our own home) before and after for as long as possible. My mom is really good about recognizing and even helping to facilitate time with DH, etc. When she comes to visit now, she'll just take our son and say, you go have time to yourselves, I'll make dinner, whatever...she just handles it, which I like.
I guess it just depends on your relationship with parents. Having a toddler already, I imagine it would be even more useful to have them around for quite a while. 2 weeks is probably the norm for most people, so no one gets tired of one another and you get your privacy back, but for me, I'd say more like 4-6 weeks.