Some dishwashers have what is called a thermal hold. This means that it requires a certain temperature before it moves to the next step in the cycle. If your cycle requires 160 degrees and your incoming temp is 120 then calculate about 2 minutes per degree so that it can get up to the correct temp.
If your using a cycle that will wash & rinse twice then you are looking at a considerable amount of time before it completes the cycle. If you turn it on before you go to bed & is still on when you wake up then yes you would have a problem. If it runs up to 3-4 hours then that is normal.
A good way to test this is to run the "rinse only" cycle as this cycle doesn't use the heating element. It should complete in the time that is mentioned in your owners manual. There should be an asterisk in your manual that covers the temperature hold.
Hope this helps.
rod
April 2008