I don't have first hand experience with that, but it seems that might actually be a poor way to learn, because everyone will want to speak English to you. I've had to selfishly refuse to speak English to people many times.
Everyone in your class of course, but its not like you're wearing a tag that say "English Teacher" when you're just walking around.
I also bypassed this pretty easily, my extended family / name is Italian, so I would always respond back in Italian, and then we'd revert to Chinese pretty quickly because NOBODY speaks Italian over there.
Further since you're there on a work visa, you can eventually transition into translation work / etc. to really refine your language mastery. 3 months isn't bad, but I'd recommend a solid year. Following my time in Asia, I lived in Russia where I basically didn't use my Chinese for a couple years, but on a quick business trip to Taipei my fluency was there when I needed it.
I'd say to master any language there is no period of time that is sufficient. I've even gotten worse at speaking my native language since I speak Spanish all day.
Like with exercise, or brushing your teeth, once you stop doing it, it will get worse and you will lose "it".
