Many factors influence how long an individual amaryllis will bloom. How much light, water and fertilizer each amaryllis receives dictates just how long it will bloom. Age is a factor, as well. The older the bulb, typically the more floriferous it will be.
Once your amaryllis starts blooming, you can prolong the length of bloom by moving it to a cooler location. A spot between 55 degrees and 65 degrees F. will keep your amaryllis blooming longer. The cooler air slows down the senescence of the flower.
After the initial bloom, you can encourage your amaryllis to rebloom. Cease all watering and fertilizing for eight to 10 weeks post-bloom. Remove the leaves and stalk as they wither and die. Once you see new growth, repot the bulb into a slightly larger container and resume watering and fertilizing as before.