Well, I'm not sure. I'd guess that perhaps your previous experience that was almost identical but not quite the same had much to do with it. I think remembering the past experience and comparing to the current one helped you pick out the inconsistency (that being the field instead of the bus) and that snapped you into lucidity.

As far as time frame, I'm not a natural (someone who lucid dreams without working to do it) so I'm not sure when most of them begin to be able to do it, but I would guess that now that you have had your first, you are likely to have more.