You can start selling them as prints immediately if you want. I know a seller who hasn't sold her original painting but has started selling them as prints and is making money from it that way. The prints will usually sell for less of course, but in time, you can wind up making possibly as much as or even more than your original did. Experiment and see how well a print will sell. If it becomes popular, keep doing it. Or you could make it a limited if you notice it's not selling, and when your inventory is sold out, that's it. You can always choose to offer it as a print again later if you want. You're the artist, so it's all up to you what you do with your pieces.
The only pitfall in my opinion if someone starts copying your painting. There's a lot of copycats in the art world today, so just be careful to watch for that.
You can try listing them in different sizes to see how well they sell. I've seen some around 5" x 7" inch size sell too. It's all up to you. If they don't sell, then you can always go back to normal size.
