there isn't any timeline, the only person we are aware of that is left while Shep is on the Citadel is the main guy who tells him the Citadel hasn't fired. We have no idea how long Shepard was unconscious before stepping into the beam.
A full retreat could have been signalled and the Normandy along with loads of other people fled, its stupid but its possible, only because the reapers magically let a crapload of shuttles land on earth without trouble, if that is plausible(but a massive stupid plothole) then there's nothing stopping the remaining crew going back the same way and them heading to a regrouping point in another system.
Its still daft, you can spin anything, it doesn't matter, what matters is how the player feels/reacts AS HE PLAYS it, and as I played it nothing made sense, it was irksome and stupid. Simple as that, they can release a DLC that "fixes" everything by explaning it a la Lost..... badly, it won't change the fact that I played 20 mins of utter turd at the end of well over 100hours gaming across the trilogy, expected an ending and didn't get one. Nothing can stop that from not having happened, so utter failure, nothing more or less.