Dbol kicks in right away. It is short acting, after 3 hours half of it is out of your system.
The reason that deca takes longer is that it is a long acting drug. It has an ester attached to the nandrolone molecule that causes it to relaease very slowly, as if when you take a dose somebody has attached an I.V. to your arm with a slow dripping bag that will take weeks to empty into you. Thus, only a fraction of the active drug from the shot is actually working in your body at any one time. Thus, it takes several shots to get enough "I.V.s" flowing for the levels of the active drug in your blood to get high enough to be effective.
This is why it is a good idea to "front load" long acting drugs like deca. If you triple the normal dosage in the first week, your blood levels get high enough to be effective much more quickly, and then the subsequent injections maintain the blood levels for the rest of the cycle. This way, less of the cycle is "wasted" waiting for the deca to "kick in".
If you do a search on front loading, you want to reach posts by Andy13 and Zyglamail.