expressing time

[soidisantfille.livejournal.com]

One thing that has always confused me is how to express duration of time in Russian. As in the following situations, off the top of my head:

"I have been a student here for five years." (I'm cheating a bit here; I need to write a short 'biography' of myself in my дневник for homework, which is what got me thinking about this.)

"I need to finish reading the book by Friday."

"It took me six hours to write the assignment."

"I will be there in ten minutes."

I know that duration can be expressed with 'на' and 'за', but I can never remember when to use one over the other. What's the difference?

Thanks.