"even at FTL speeds, it could be months or years before they reach us" - according to Amanda Kenson. as far as the DLC itself goes, there is no set time distance between the destruction of the Collector base and the events of Arrival, which means this has no bearing on the ending cutscene of the Reapers approaching the galaxy.
even though there's indeed no in-game clarification, based on the in-game information it's logical to assume the Reapers started heading towards us long before the Collectors got their candy bums walloped.
Right, the game does not set a fixed time between the arrival DLC and the destruction of the collector base. It would make the most sense to me to start the arrival DLC directly (or some weeks or something) after destroying the collector base, but that is not enforced. Then, the reapers are shown to start their voyage after the destruction of the collector base, but of course that does not necessary imply when that happens.
I would argue that the narration has kept a strict chronological structure so far, so obviously players will assume that the reapers start after the suicide mission, but it's not like we see a clock or something
.
Now what happened is Sovereign sent that signal but the keepers ignored it. So Sovereign had to get other help. It got Saren and the geth and probably started the collectors up. Now Sovereign is gone so they have to resort to plan B and C.
According to the point made above, we can construct our head canon in a way that plan B and C are executed simultaneously and are not exclusive. But still: What is plan B, then?