It is valid from the start and immediately effective.However, both people have to physically meet to become conscious of its effects (if they haven`t met and only one person is conscious of the other, this person will still feel it, but of course in a lopsied way, as it is of no concern for th eother person - yet).
IN fact I noticed several times when I came into physical proximity to people (I previously just know of before), it felt like something shifted and locked into place.
Also usually you can see a first meeting quite clearly in the composite, too.
At the day of meeting someone pr composite Venus had just come into exact conjunction to composite Antivertex, and Tr Neptune and NN was conjunct pr c - Venus and c-Avx. Along with Tr Jupiter conjunct the composite Moon and Tr Juno conjunct the composite Sun exact.
When we met again, pr composite Sun had come into precise conjunction to composite ASC. (orb was 0°01!)
in three cases I had a pivotal moment (first seeing someone in th ephysical) when Tr Pluto was exactly conjunct composite Sun, in the fourth case it was when Tr Neptun was conjunct composite Sun (and Tr SN-Saturn exactly opposed the Venus-Jupiter-conjunction in composite)