How long does a game of risk take? ?

Note that it can take longer.
Our first game was about 2.5 to 3 hours; two of the players were basically new to Risk, everyone was new to a "modern" Risk game, Ours took about 2.5 hours because there were some new players and only a few had read the rules.

So maybe 2.5 to 3 hours I guess :)
Make sure all of them read the rules and understand it,
So the game goes faster.
Have fun