RMB 30 card gives you 4000 minutes (66.67 hours), which tends to last me about 2-3 weeks, although there are many who play a lot more than I do and probably use it up in a week.
I wouldn't worry too much about the other teachers expecting you to work, work, work, as they're probably used to foreign teachers and their smaller workloads. You're there mainly to be a celebrity rather than teach anyway, right? ^_^
Also, you can create 2 wow accounts under the same ID card, so you only need to find a single ID card in order to let both you and your girlfriend play.