Actually HEED is for the first two hours. From the Hammer product page: "HEED may be used as the sole fuel during exercise lasting up to two hours, fulfilling all your calorie and fluid requirements..."

I use Hammer pretty much exclusively. I make one "two hour" bottle of HEED (five scoops / ~500 calories) before I start riding to be consumed over the first two hours. For a longer ride (4-7 hours or a century, etc) I make up one or two "two hour" ziploc bags of Perpetuem (four scoops / ~500 calories) and stuff them in my jersey. When I finish the first bottle just find some water and dump the contents of a baggie into the bottle.

I have my consumption rates pretty wired but this methodology really helps me maintain my hourly caloric intake. I know that every two hours I should have gone through a bottle of mix, HEED or Perpetuem and at least half a bottle of plain water. No thinking required.

I also carry a 5oz gel flask with Hammer Gel (again about 500 calories) for the occasional climbing jolt and as a fallback - if the ride goes much past 6:30 you have two hours of calories in the flask.