According to Yoshinori Kitase,
Final Fantasy XV should take a much shorter time to develop than Final Fantasy XIII. GamingUnion had the chance to talk to Kitase at this year's GamesCom, and posted the following revelations:
"Obviously we now have consoles with much better specifications. The graphical quality we should target at is a lot higher, so it takes a long time to achieve that quality," said Kitase-san.
He went on to say "And Final Fantasy XIII, for example, was the first ever numbered Final Fantasy for Xbox 360 and PS3, so we really wanted to make it absolutely perfect and it took a long time to make it perfect. But maybe next time around we need a much shorter time."
We then quizzed him about his final statement and asked if that meant the next main game will take less time to complete. He simply replied by laughing and saying "Yes, yes, hopefully."
Source:
GamingUnionUpdate (Dec 2, 2011): It now seems that Kitase was probably talking about XIII-2, not XV. At the time of this news post, XIII-2 hadn't been announced.