Crappie in the tank

My sister has a fish tank. She has some goldfish in it, but they died.
So I bought her some more goldfish. They died too. So I figured, those
critters are not that cheap. So I went and bought her a couple dozen
crappie minnows. And they ain't died. They school around the tank and
do things. Nothing at all like boring goldfish.

My only concern is, how long does it take for them to grow up? The
first batch of goldfish had one straggler who managed to live for 8
years. It was pretty big. How long do you think we have before we have
to start thinning out the school? It's a 55 gallon tank, there are
about 2 dozen minnows in there now. They are about an inch or so long.