Well the timeline here
http://dragonage.wik...m/wiki/Timelinesuggests that humans first explore Thedas at -3100 ancient, which is 3100 years before the chantry calendar starts. Since until the veil apeared only Dwarves and Elves lived in Thedas we can assume it is some time before this.
Since then there has been 9 ages - I think that's another 900 years - so my guess would be that Solas created the veil between 4000 and 4500 years ago.
Deducting the times in the elven calendar gives 3904 years between Inqusiition and the first humans