nandrolone deconate has a halflife of roughly 2 wks. So, it will take a few weeks for your blood levels to stabilize. This does NOT mean that you won't get any effect until stable blood levels are reached. ANY injection starts working from the moment it hits your blood stream. What you've noticed so far MAY be placebo, but the science says that the deca is already hitting your androgen receptors.
as for deca dick, it is not a certainty. some people experience it, some dont'. its usually more prevalent when peopel use high dose deca and use for long periods of time.