It depends on a lot of circumstances including the humidity in the air, the amount of leaves that are on the plant, where they are in their dormancy cycle, how big the bulbs are and what type of medium they are potted in. The simplest answer is to water them whenever the soil feels dry to the touch.

When they are actively growing new leaves or stems, check quite often because they needs water to fill the expanding cells - during those times you can water even if it's not completely dry, which might be every 5 days or so. During this time I also let water sit in the tray and it gets drawn into the pot usually in less than a single day.

When the leaves are mature and the plant is no longer blooming, water when it gets dry to the touch which might be every 2 weeks. When it's going dormant or has gone dormant, wait a little longer even after it's dry; it might be 4 weeks or you might water it just a single time during dormancy.

Hippeastrums resent too much water and will do better getting a little less than they like than they will getting more than they like. They are good for people who often forget about their plants. The worst that happens is that the flower scape is shortened due to being too dry, or the bulb regenerates its size slower than it would otherwise.

Thanks, pizzuti. My two are actively growing so I'll just have to time it well and hope I don't cause any rot.