This is a common question I get over email so I wanted to write a post that covers the issue. I'll add it to my email series as well. It's an important question because it can save you a lot of time in the process depending on how strong the liquor is that you're using.
Stronger (higher proof) liquors will do a faster, more efficient job of pulling the lemon oils out of the zest. Therefore you can shorten the infusion times if you develop an eye for when your zest has fully given up the ghost.
This is easier to see if you have zest at several stages of infusion, but I wanted to give you a benchmark for what zest looks like when it is finished. Below is an image of the finished zest in the alcohol. It's a bit harder to see that it is as pale as it is when it's in the infused liquor because the yellow color is now in the liquor itself.
Here's what it looks like out of the infusion. As you can see, it's very pale in color, almost a sickly grayish tint. The shade varies a bit, sometimes it's more whitish, but you're just looking for the distinct lack of the vibrant yellow color that the zest had to begin with.
Below is the real close-up and you can tell here that the zest is pale and translucent. There's still a small amount of yellow left but this zest has given up every bit of lemon oil that it has to give.
And now you know! If your zest is pale and translucent, it doesn't matter if it's been sitting there for 30 days or for 45 days. You can stick a fork in it, 'cuz it's done.