How long does it take for a snail to get as big as the eraser on a pencil?

I got some plants in november and they must have had snail eggs in them.Now i have this one snail in my 20g tank.what should i do? I don't mind the snail in the tank.it has gotten three times it size in a couple of days. I don't know what kind it is.thank you all.|||two months , maybe?|||About a month