From my How old will you be topic:
Here are the dates of Gran Turismo titles first released in Japan.
Gran Turismo (Japan): December 23rd, 1997
1 year, 11 months, 18 days later....
Gran Turismo 2 (Japan): December 11th, 1999
1 year, 4 months, 18 days later....
Gran Turismo 3 (Japan): April 28th, 2001
3 years, 8 months later....
Gran Turismo 4 (Japan): December 28th, 2004
5 years, 1 month, 26 days and counting.....
Gran Turismo 5: Still TBA
Take a look at the time between GT3 & GT4 (same platform: PS2): 3 years and 8 months. That, too, is quite a long wait. Now, speaking of GT5 & GT6 (assume the same platform: PS3), I don' think it'll be nearly as long. If GT6 appears on the PS3, I'd be the first to guess it'll take between 2 and 3 years. Better yet, maybe even 1.5-2.5 years. Because afterall, the content difference between GT3 & GT4 was absolutely HUGE. From a mere 20 tracks to 50, and from a car jump of about 300 to 750. Not to mention, a notably more in-depth GT Mode. I would not expect a content gap as big as that regarding GT5 & GT6. Hence, less than 3 years. Perhaps about 2.