A two to four second delay is normal, not only for Dish, but for Digital Cable (when tuning a truly digital channel) and DirecTV. Anything longer than that could be a problem. My 721 takes three seconds or so.
The reason for this delay is that the box not only has to tune to the channel, but it has to reconstruct the bit stream into picture and sound and synchronyze everything. The old analog signals were all there the instant the channel was changed so you didn't have to wait with analog Cable or regular TV.
A receiver cannot reconstruct the whole thing and synchronize the sound properly until there is a reference frame or full frame. The audio could come a lot sooner than the picture and that is what some cable companies do when you change the channel on their digital boxes. The audio pops up within a second and the picture reconstructs itself from little squares until there is a reference frame and bang, the whole picture shows up.
Dish tried something like this a long time ago where the sould and picture would pop up as soon a it was possible with MPEG and then synchronizes the sound and pucture. However this lead to the complaint of the "frog voice" for the second or two it took to slide the audio in sync with the picture.
DirecTV's boxes seem to be a little quicker at putting up sound and picture and they more seamlessly slide the audio to match the picture. The difference is that with Dish the boxes now "wait" until the have everything then put up a pucture while DirecTV and some cable companies contruct the signal while you wait. The perception of time is an important factor here. Even though it takes a frew seconds, watching a picture construct itself before your eyes is "okay"...a blank screen for the same period of time is unacceptable to some and perceved to take longer.
Anything longer than a 2 to 4 second delay is a sign of other problems perhaps switch or signal related.
See ya
Tony