9 days
if ya have a few hundred DMD and leave ur wallet open in minting mode

u will mint more often some DMD

at the begin it will be just once every 9 days but as the coin piles split down u will see minting happen more often

but as always with such things more DMD create more POS rewards 

i would say minmal amount of DMD that make sence is 100 DMD
that will generate u 4 DMD a month minting

now multiply that with what u able to afford (willing to invest) and u know what earnings u can expect