Hmm, I've seen a hack that shortens the duration, but now for the life of me I can't find it. In the mean time though, you can force the Gypsy to deliver the lamp whenever you wish by using the testing cheats.
(boolProp testingCheatsEnabled true)
Shift-click on the mailbox, choose *other (I believe) and choose 'Force Genie Delivery'. I'm not sure of the exact wording, but it's pretty obvious when you see it. Within an hour or two, the old crone will drop off your lamp and skulk away again. You'll even get the memory. Best of all, you can do it as often as you want.
*grabs her cat-o-nine-tails* Now, about that whipping...
