How much tea is ideal out of one tea bag? My mother regularly uses 1 tea bag for two cups or 2 tea bags for a teapot. I've read on several sites that one tea bag is ideal for one cup.

So, how much should I get from a tea bag?

Also, is the steeping time different if I was trying to get more tea out of the same tea bag? Some say that steeping it longer gets more flavour out of the bag. Somewhat more formal research suggests that releases the bitter parts and makes the tea bitter rather than thicker.

I know you shouldn't reuse coffee beans when making an espresso, but does this apply to tea as well?
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.