What is a reasonable amount of time for my in-laws to stay at my house when they visit us?

They live 1.5 hours away. We have a toddler and baby on the way and I’m tired of hosting them. After I see my MIL I feel horrible about myself because she’s just subtly mean. But my son’s birthday is coming up and I know I’ll have to host them for some period of time. They usually stay from 10am-7pm and it just feels like too much.