Lilium leaves age naturally from the bottom up. You can usually expect some lower leaf loss during the growing season.
Greene is correct in that overwatering will hasten this loss. You want to keep the leaves as long as possible so as to bolster the bulbs for the next season. Also keep in mind that too much water plus high heat can result in bulb rot and loss of bulbs.
Lilies need some water up till bloom time then cut back as the plant rests for the remainder of the season. The growing mix must be free draining so that when you water it runs right through the mix and out the bottom, leaving the mix just damp. Oriental lilies are the latest lilies to bloom, I'm guessing around February in the southern hemisphere once established in the ground. So they will need water up until then, then infrequently afterward.
Lynn is correct about scorching heat/sun. Oriental lilies in particular dislike high heat. If your summer is very hot (90F, 30-35C) you will want to plant these lilies out where they get just morning sun or dappled shade. They like at least one half a day of sun to do well.
They do not need any fertilizer now. These florist lilies have already been prepped with all the plant food they need for now. Also too much nitrogen will cause bulb rot. I fertilize my lilies when they emerge from the soil in the spring and again just before the blooms start to open. Using a low nitrogen fertilizer is best. I use a granular vegetable fertilizer (5-10-10) when the stems are just a few inches out of the ground. About a tablespoon sprinkled on the soil over the bulb is enough, then water in. It is slow release so it won't dissolve all at once.
