About 6-7yrs, when I got him he was about pencil size. I was a student then, with more time than brains/money, so the tank got a lot more attnetion, including frequent trips to the compost pile for worms. He ate well.
I imagine it would take much longer with more reserved feedings.
As long as there to big to fit in the eels mouth it should be fine. They do get very large and are a rather aggressive eater. Also I wouldn't put them in with some of the really aggressive or nippy chichlids. Hope this helps.