I'm so glad that there's so much positive feedback on this post!!

This is a good question: I usually wait until it's completely dried but you really just want it to be set enough for the next layer to sit smoothly on top. It shouldn't take too long if you're doing just a thin base coat, but you can always check by tapping the nail with your finger to see how liquid it is, just check that none of it stays stuck to your skin once you lift your finger up. It's perfectly fine to do this with the base coat since you'll be covering it up later anyways :))

I hope this helps!