What you're looking for with cracking WPA-PSKs is handshakes (clients joining the network), so the exact amount of time that you'll need to wait depends on how busy the network is.
As a tip you can run airocrack-ng on the cap files whilst you're capturing traffic and see if there are any handshakes listed for the network that you're interested in.
Also another way to speed things up is to lock airodump-ng onto the channel that the network you're interested in is using (using the --channel switch) as that way it won't hop around all the channels and you'll get more traffic that you're interested in.
Additionally if there are clients on the network but there's not a lot of joining activity you can use a deauthentication attack to speed things along.