PDCs are ground based for players - only the AI uses orbital "pdcs" (actually just ships/stations) due to the issue of having to write separate AI text for just that I presume.
I have never increased deployment time for them over the minimum, and you are right that it will always stay at 0 since they are on colonies. However, I do not exactly know when a colony really qualifies as a colony, so it could be that PDC on a planet that has nothing else anymore (no population, infrastructure or factories/mines) will not receive this. Seems like an exotic situation though. Have never had this yet, though I plan to.