Last time, I plugged my evo wingle in 10AH power bank and that gave me straight 30 hours of backup. So, power banks are excellent for getting extended backup when other USB power source is not available.
Zong Mifi gives 3-4 hours backup if you use it for heavy downloading. If put it on standby and use internet occasionally, backup extends to 5-8 hours. (Observed personally) If you get poor signals, battery will drain in no time.