1) If you're able to find 100 proof vodka or higher, use that. The higher the proof, the more flavor will be extracted from the lemons. If you can only find 80 proof, then just allow the peels steep for at least the full 7 days.
2) When peeling the lemons, be sure to remove all of the white pith. This is bitter and will transfer to your limoncello if it is not removed.