I would test the water paramaters and use that as your guide. I would say a few day's after the first cycle. Make sure to properly acclimate himas the water that is just cycled will be a big change from any in a lfs.
I know this may seem completly off of what others would say... but to tell you (from my own personal experiences) ive put tangs in a tank only 2 days after i set up the tank and they did just fine... one was a yellow tang and the other was a sailfin, I think the reason they did so well is that i used cycle (a live bacteria additive that puts the right type of bacteria in the aquarium to get things started well!) I think that tangs are alot hardier than others make them to be on this message board...
well, sorry to tell you but, i would put NO tangs in a 29 gallon tank, at NO time, if it is algae you are looking to rid, i would use inverts and maybe a bleny(lawnmower)