Guys,
the different recipes use boil times and % values for the hops to add.
% means how "bitter" (Alpha Acid percentage the hops contain) the hops used are, so if they say "20 gr of 6% Saaz" and you have 3% Saaz you would use twice as much as the recipe says. (when buying hops in 100 gr tubs or more it says the % on them.
the boil time is counted from when you finish the boil, so not from the start.
This is because the longer they boil ,the more bitter the beer will be.
So a 5 minute boil means to add the hops at 5 minutes before you plan to finish boiling, then fish the hops back out (at once, or after having chilled the wort)
A 30 minute hop boil would mean just that, you get a rolling boil, add the 30 minute hops, then another hop bag at 10 minutes, 5 min or whatever the recipe says.
The different times gives different flavor parts, such as aroma, "flavor" or bittering,
this is a great read if you want to do more:
http://www.howtobrew.com/section1/chapter5.html
thanks
Bjorn