Hi,
you just need to use the formula (this can be found in page 18 of datasheet):
It is you who need to define either t (time duration of the sweep) or DFTW (the frequency increment).
So say you have DFTW = 1MHz, SYSCLK of 1GHz, DFRRW of 65535, the time duration of the sweep would be 22.52ms.
t is needed because the sweep operation in AD9858 does not have automatic stop-at-a-frequency.