Based on anecdotal evidence, Int is one of the faster trades for promotion. I was on course with a bloke who made Sgt after 7 years. That said, he had been awarded an MBE off the back of his (very sneaky beaky) job. Of course, promotion runs in cycles based upon the demographics of the people within the trade, so you could have a few years of very quick promotion followed by a few years of tumbleweed.
In most trades (not just Int) it is certainly possible to do 3 years at each rank nowadays i.e. promote after every tour, but you have to put the work in, know what the promotion board are after, play the system and have a bit of luck along the way.