I boil down 2 gallons of first runnings down to about a quart - a nice thick brown syrup. The best way is to get a much deeper (read that as "BIG") pot than you think you'll need, since as the wort boils it foams quite a bit, and gets slower and slower to reduce as time goes by. In my experiences it takes longer than 90 minutes to get it boiled down enough to create the caramelized syrup that, once re-added to the boil, will provide the brewer with the great signature flavor you should be (IMHO) looking for.
I run off the mash into the syrup pot and the rest, plus the sparge, into the kettle, and actually start boiling the syrup for 1/2 hour before even beginning the kettle boil. For me that works about right, although even this way I've gone a good 2-hour boil total for the full volume boil.
Good luck. It's worth it - a great beer.