if you get it early they can always come back in and give you more medicine. you can also ask your nurse if she thinks it's a good time to get it now. I was told the other day that if I'm around 3-4'm that I should get it and not wait, since I get induced tomorrow night. so, do what you feel makes you comfortable and you know that you will get some rest.
im just going to say get it when you want but getting it early can mean it can wear off when it gets hard or waiting too long can mean it wont kick in much by when you need it. just sharing these from lots of moms experience.
I got it at 3 1/2 but my water was broken and I was asleep right after. i got it around 10 then was woken up to push around 1230 daughter was born at 107 am. she had complications though.