Question: The specification says the time must be less than 1 character time, but no tolerance is specified. 

0.5 (4ms) character is a reasonable time that all DUT should recognize as permitted. The tests expect the device will answer. 1.5 (14 ms) character is a time that no DUT should answer. A Gap Time of 0.5 is needed to pass the test. It is a reasonable expectation to use 0.5 as Gap Time. Gap time of 1 character time or greater would be considered a failure.

See Slave Token-Passing Data Link Layer Test Specification, Test case DLL013 FSK Gap Receive Timeout Test.