You need a 60min boil for AG, more like 90min if you are using pils malt. You can add hops at any time after the boil commences, it really just matters becaus ethe longer you boil the more bitterness you get. For instance a 60min boil might give you 24% conversion and a 90min boil, 27%. Not a huge difference but knowing this you can choose when to add the bittering hops. Also, the later in the boil you add hops, the more of the floral/citrus/pine flavor/aroma compounds remain. Thats the purpose for late additions (15min, 5min), but some people like to do their bittering addition at 30min left in the boil to get added flavor. You sacrifice some of your conversion of alpha acids but a lot of the new hops are super high so you still get plenty of bitterness plus the extra flavor.
Check out “How To Brew” online for a good discussion on hops and when to add them.