Well, according to the documentation
"The value must be an integer, such as "100". Higher numbers have a higher priority."
So I'm guessing that any integer value is valid. And the highest priority possible would be the maximum integer that android allows, which is (2^31 - 1).
EDIT
The documentation has been updated, and it now explicitly states which priority values may be used by applications. The documentation now says
SYSTEM_HIGH_PRIORITY (1000): Applications should never use filters
with this or higher priorities.
SYSTEM_LOW_PRIORITY (-1000): Applications should never use filters
with this or lower priorities.
Implying that your application is permitted to use integer priority levels between -999 and 999.
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