For best data collection I use --ivs --channel and --bssid options for airdump.
I collect about 300000 ivs. Collection time for ivs depends traffic for the wireless network.
And again it took about a second to find the key I used.
For more security with WEP use dynamic key provisioning. 128 bit key or more. Then the attacker has to collect more data to hack your network.
Also use MAC filter and disable SSID Broadcast. But do not forget it is possible to change mac address and find your hidden SSID for an attacker.