task in PIC16f887 with watchdogtimer
The watchdog always runs from the internal 31KHz internal oscillator.
The general formula for calculating the watchdog time is:

\$\frac{1}{31000}\times WDTPrescaler [\times Prescaler]\$
Where \$WDTPrescaler\$ is the dedicated 16-bit watchdog prescaler, and \$Prescaler\$ is the optional 8-bit prescaler shared with the Timer0 module.

So in your case it's

\$\frac{1}{31000}\times 128 = 0.004129032\$

Which is roughly 4ms.
If you were using the secondary shared prescaler you would multiply the answer by that value as well.