Well I've abandoned my search for a proper job and just come back into the lab so I can answer this question.
Boiling wort essentially serves 3 (perhaps 4) purposes which are time related.
1) Isomeration of the hop alpha acids . . . which makes wort bitter
2) Sterilisation of the wort
3) Reduction of protien, through heat degradation.
4) Drives off unwanted volatiles
Extraction of bitterness is related to the amount of hops, how much alpha acid they contain, and, duration of the boil. If you reduce one of these, say boil duration, you have to increase either one or other of the two factors . . . most easily by increasing the quantity of hops. There are several formula that you can use to determine thins adn they are incorporated into most brewing software.
Sterilisation of the wort is achieved by a boil of 15-20 minutes, so all worts should be boiled for a minimum of 20 minutes. . . . even kits technically.
Unwanted volatiles can be driven off quite easily. The one the US brewers all seem to worry about (DMS) does not exist in British malts as it is taken care of in malting, so a 60 minute boil is adequate to drive off any left behind following malting. Foreign malts may be higher in precursors so a longer boil may very well be desired.
The real factor that determines a 60 or 90, or longer) boil is getting rid of excessive protein.as the boil proceeds protein clumps together and the wort gets progressively clearer, eventually there comes a point where the wort does not clear any more and the protein clumps are about match head sized. This is the hot break, and generally never happens before 60 minutes . . . but more usually 90 minutes. I therefore prefer to say boil to secure the hot break, rather than giving a time to boil for. If you boil 'hard' then you can achieve the hot break sooner, but you have to then accept the issues that result from excessive thermal degradation of the wort, a more gentle rolling boil achieves the hot break after a little longer boil. For those especially pale beers wort is boiled at a 'simmer' and for 2 or three hours!!
