TPS92518HV: what is the minimum off time which keeps TPS92518 in proper operation

The minimum off-time is not controlled by or specified for the TPS92518 device.  As it is a timer that controls off-time, there is no internal blanking circuit that establishes a minimum off-time. 

Functionally, with any buck converter that implements a high side MosFET gate drive, the minimum off-time is governed by the minimum amount of time necessary to charge the bootstrap capacitor to ensure the high side MosFET can be properly driven.  In the event that your duty cycle becomes too high to ensure the minimum off-time, the BOOT voltage will fall (over several cycles) until it hits BOOT_UVLO (typ 4.4V) and the high side GATE will then turn off and the off-time will last long enough to charge BOOT back above that threshold.  If you remain at this operating point, the converter will cycle in and out of BOOT UVLO continuously at some effectively subharmonic frequency.

Regarding the absolute value of off-time where this occurs, that is a function of the switching frequency, operating points, and the BOOT capacitor value, as the on-time will deplete the BOOT capacitor and the off-time will charge it.  And given this is a variable frequency device over the operating range, the frequency will be at it's lowest at the highest (and lowest) duty cycles.

So, there is no one easy answer to this question.  My suggestion would be to experimentally determine whether you are discharging the BOOT capacitor too far at your worst case (highest) duty cycle.  I would use the BOOT UVLO max spec (5.2V) when observing this and test it over temperature.

-JP