How much tea is ideal out of one tea bag?
This completely depends on the quality of the tea and flavor you're going for. It is common practice (especially in China) to make 2-3 or more cups/pots of tea from good quality loose leaf green tea. The flavor will change after each steeping, becoming lighter and milder, but also just different in certain ways. You can do the same thing with any lower quality tea, you just have to decide  if you like the flavor of the second, third, etc. steepings. If not, don't do it.