Should be every week to three weeks, give or take a litle. I would expect sooner than later though, Veileds seem to grow a bit faster than the other more commonly kept species. That being said, every chameleon grows at a different rate, and will shed less frequently as the individual gets older.
Just for the record, i have never had a veiled, i am just going by what ive seen during the almost unnecessary amount of time ive spent on the forum But im sure someone will correct me if im wrong hope that helps!
I have had one veiled as a baby. I got her at 3 months and she shed every month until she reached 6 months, then she shed every 3 months after that. As they get older it takes longer to shed all the old skin - babies can pop the shed of in a matter of hours, but adults may shed one body part at a time and it will take several days for a full body shed to take place.