There's no 'right or wrong' time IMO... it's whatever you & your LO are comfortable with.

I left my older three kids with my mum quite early, we lived with her or near her & they saw a lot of her so it wasn't a big deal.
DD2 is 9 months old now & I haven't left her overnight yet, nor do I want to any time soon - in fact DD1 has offered to have her in her room overnight & I can't even bring myself to that

I think it's a nice idea that your DH has had & probably great for you as a couple, but won't be much fun if you are worrying about how your DD is going or pining for her.
Can you do a dry run first? Maybe work up to it slowly...I don't know if you already have or not but leaving her with someone at home while you go out to dinner might be a way of seeing how she will settle for someone else (and at least you are close by if needed).

And if you just don't want to & don't feel comfortable yet, then don't