It depends on which interrupt you are triggering.

For INT on PORTB.0, it's a schmitt trigger input.
For PORTB change on RB4-7, they are TTL levels.

Levels for both types are listed in table 17.2 DC Characteristics as VIL and VIH

And yes, TTL low is .8v, high is 2v (assuming 5V VDD).
Schmitt trigger is 0.2 * VDD (1v) and 0.8 * VDD (4v).

hth,