I would think you could plant it now if you wished too. They are usually in pots much too small and could do with a bit of foot room. Or, put them into a larger pot and then bury the pot so you can lift them up in the winter season later. Lillies really don't tend to be very pickt but you get larger plants and more blooms if you feed regularly.

I recommend to plant a few together, so if you have time, see if those same stores are now selling them on clearance (or give a little more time for the flowers to drop off and make the plant look like a clearance plant). Plant them together at that time.

If not, then just what you have should be fine to plant now, or after the bloom is off.

make sure they get a lot of sunlight, and keep them away from your other lily. lily virus is prevalent in easter lily from big stores. it may or may not survive since it was forced to bloom early.