So i kinda need to know how long the crusade was going on before the primarchs were found or just how old Terran Veterans were mainly

The Crusade began when the legions left Terra. They found Horus within 14 years, IIRC?

The Heresy began just under 200 years after that.

We may get more context on the number of found Primarchs at a specific time in the Crusade, during the Gedren event... ;)

You think it would have been a longer dater like 2,000 yrs instead of 200
yrs of the Great Crusade it seems longer.

The Crusade lasted 200 years from when the Emperor's forces left Terra - it has been pretty much a constant ever since the background of the Great Crusade was first mentioned. I don't recall any background mentioning how soon into the Crusade Horus was found, but I could be wrong. We do know, however, that it was 30 years before the next Primarch was found (it is mentioned in the Luna Wolf Index Astartes).