90 Minute Boil
For the beginning brewer, a 60 minute boil does the job for the most part. Extract brews generally will not contain the compounds generated by the mash process as the extract has been boiled already, and the 60 minutes is enough time to properly utilize the hops and remove any off flavors. That said, all grain brewing, just like the pros use, really requires a 90 minute boil. Different grains and mash techniques will create different amounts of compounds that will be removed in the boil, but 90 minutes will assure that this happens most completely, where 60 minutes will not. DMS or Dimethyl Sulfide is a compound that is present in hot wort and is removed by a good rolling boil and evaporation. The flavor and aroma is similar to cooked corn. When making ales, some of the remaining DMS will be removed during the vigorous ale fermentation, while lagers need the DMS removed in the boil as the fermentation is less vigorous. In fact, 100 minutes of boiling time is really necessary to remove most of the DMS in the boil (keep in mind, the wort should be allowed to evaporate and not be completely covered or the DMS will be trapped in the wort). This is where the 15 minute pre-boil comes in to play prior to adding the bittering hops, giving a total boil time of 105 minutes. Often when shooting for higher gravities, there still will be sugar available to sparge from the grain bed when the proper volume of liquid is reached, allowing for a little extra collection. The boil will concentrate the wort over the longer period and I have found it helpful in hitting SG targets.
Hop utilization is increased with a 90 minute boil, not by a whole lot, but enough to help decrease the amount of hops needed to hit the IBU targets for the recipe. Another reason for doing the longer boil.
Another little tidbit is clarity of the finished product. A vigorous boil is required to coagulate proteins in an all grain wort, especially when high protein malts are used such as wheat. Allowing the wort to boil for 15-30 minutes prior to adding hops will help this coagulation of proteins. This 'hot break' can be seen forming on the surface of the wort as it boils. This pre-boil is the German method of brewing, for those interested.
An important note, mentioned in the "What You Need" section, is the heat source. Electric stoves will effect the beer while boiling by caramelizing or burning wort due to the intense heat of the heating element. If an electric stove is the only option, then use something like a wire tivet to place between the pot and the stove coil. This will help reduce the problem.