Boil times?

[Legman]

Most recipes I've seen use a 60 min. boil
But from time to time I see 90 and 120 min. boils.
What is the purpose of these longer boil times? What do they really do?

[slothrob]

A long boil can accomplish a few things:

Intentionally caramelize the wort for a Scottish Ale.
Concentrate the wort for a high gravity beer.
Blow off DMS, particularly in a light colored beer made with a lot of Pilsner Malt.

Perhaps there's more reasons, but those come immediately to mind.

[coder]

I usually do 100 min boil (90 min plus 10 min pre-boil before I add bittering hops). This time is best for most beers, 60 minutes is often not enough to dive-off DMS and other unwanted flavours, especially when you use lots of pilsner malt and you have no posibility to cool the wort down quickly.
But long-time boils have also some disadvantages: wort darkening, kettle camelization, excessive melanoidin formation, so when you make a delicate beer, don't boil too long and don't use too big flame.