According to the datasheet, the wdt oscillator is the "LPOsc" as far as I can tell.
And that has the following specs:
So the answer must be "approx 1 MHz"
The answer that Carsten Groen gave is correct. TWDCLK is the clock of the Windowed Watchdog timer, that corresponds to the LPOsc. If you want to substitute TWDCLK for the corresponding value you can use 1MHz.