hmm, well most of mine get fed off fairly quickly since i give them to my cresteds and there are a lot of mouths there. i found a cup of them recently that is probably more than 6 months old (who put that cup in that cabinet anyhow?! ) and there were still a good number of live worms in it, so i think the question here is not how long will the worms live but how long will it take for your gecko to eat them.

the nice thing about the phoenix worms is that they don't have to eat them in great quantities because they are high in nutritional value, so i'm guessing an adult leo might eat 6-8 per day depending on the size of the worm. that's totally just a guess though, so you're really going to have to experiment and see what works for your leo. try a few at a time and see how many he can eat at once--if you leave them in the dish for more than a day or two, especially if it is hot the worms will dry up and die. definitely keep the dish on the cool side of your tank since they really can't handle heat.