My sister came to me for 2 weeks when DS2 was born but she's child-free and able to do what she likes at the moment
If the shoe was on the other foot, I would try go for a week but I'm in another country (9hr flight away atm) and I know my DH would be fine with the boys that long. Would more be a matter of could he take the time off work. I think planned in advance enough it would be OK.
So yeah, if you're close to your sister and you want to be there I'd say go for as long as your DH can be away from work! But that's just me