ha ha not long. I kept one as a kid and I think it went from about 3 inch to 12 inch within a few months, these thing eat and grow very quickly, certainly not suitable for a 20g tank at any stage
i had one in a 5 foot tank, went from 3" to 2 foot in a year, last straw when it ate my jardini, i would advise against these monsters unless its for a pond