Depends on the urgency as they prioritize appointments according to needs, so if you have other existing dental conditions that require immediate attention, you get in sooner. On the whole, their turnover rate and efficiency is quite impressive by my totally non-medically qualified take.
Besides, orthodontal work is usually not urgent. The state of your dental structure is not going to change dramatically in 6 months of delay, unless pre-existing conditions dictate otherwise.
If you're not sure which to choose, a safe approach would be to go to one of the reputable private hospitals like the Raffles group or Mt E. Generally, places like these attract (or invite) only the best, or so I was told by other medical professionals. I've had friends who went for dental work there and they are very happy. Be prepared to pay a lot though.