If you ever buy a live plant be sure to expect snails. I first bought the plant and then everything seemed normal then I saw a tiny snail on the glass and a day later nothing. After a month I started seeing small snails in my aquarium then they got larger and larger and I began seeing more and more snails. Then the entire bottom of the aquarium is littered with snails.
The snail population later died down and there was much less of a problem. The population died down because the snails had consumed their food supply and a great deal of them were dying. By this time you should no longer see any large snails. The population will then start being controlled by the amount of food available for the snails.
Some of the snails will eat your live plants! To solve this problem you will want to pull the plant out sometimes and wash it off to get snails off. You could also try to find a fish that will naturally be a predator to the snails and help control the population of snails some.
Usually it will take about a month after buying a plant to get all of this mess and then a month for everything to get under control and the population to die down. You probably will not see a snail bigger than a rasin after two months because all of the big snails will have died off.