My let down times vary with each feed, but I personally haven't timed them. I've even noticed lately that sometimes I don't feel my let down at all (even though for me its a very strong feeling)
My DD gets milk before letdown. As far as I know, there's always milk there, just that let down adds some pressure to get the flow happening a bit better. For DD it just means she doesn't have to work so hard anymore & can just lay back & relax
Your body will settle into feeding & sort itself out

ETA - my health nurse was just as interested in out put as she ever was in weight gains.