Keep in mind that I dehydrate hamburger until it is about the consistency of Grape Nuts breakfast cereal.  And if you've had any experience with dehydrated chicken, you should estimate at least twice the amount of time to re-hydrate.

It doesn't take a lot of water to rehydrate hamburger, but it does take either a fair amount of time or some heat.  

Typically, I use dehydrated hamburger mixed with other ingredients.  For example, on my last trip I had beef and bean burritos.  The dehydrated hamburger was added to the dehydrated refried beans with only a little extra water added because of the hamburger.  However, I slow-simmered so the hamburger would have ample time to rehydrate.  You can shortcut the simmer time by placing the hamburger in water prior to adding other ingredients.  I'd let the hamburger set for at least 20 minutes (length soak time by as the actual cooking time is lessened) before moving on to other steps in the food prep cycle.  I'd recommend the minimum cook/soak time to be not less than 1 hour.

dd