What is the required length of the service entrance cable into a dwelling?
All the way from the service point to the service disconnecting means.
Are you are asking how far inside the building we can run SE cable inside a building?
There is no answer, it is up to your local AHJ.
Here is the NEC 2002 section for you.
230.70(A)(1)(1) Readily Accessible Location. The service disconnecting means shall be installed at a readily accessible location either outside of a building or structure or inside nearest the point of entrance of the service conductors.
You local AHJ gets to decide how 'far' "nearest the point of entrance" can be.