15 years is about right. Most hard-core fans of any sport or team grow up with that sport or team, so you're targeting school kids. A 12 year old in 2011 is 23 now. A young adult with other interests. Give it another ten years and they'll start having kids. 15 and their kids start school themselves. It's that second generation that will start to solidify the Giants.
That the difference between heartland & one of many sports. If 15 years get a niche, its a success - it took the Swans 25 years for a flag & success is another milestone in a sporting market.
Aussies love winners.