I'd say 15-20 years is a good general guideline, as it follows the cycle of current>obsolete>revived.
The current era of gaming, typically marked by new console releases as most popular games are made for consoles first, draws attention away from the last era, as people look forward to the new thing and want to leave behind the hindrances and annoyances of the old paradigm.
But, after about 15-20 years, appreciation for the abandoned era becomes cool again, as people are able to appreciate it with hindsight they didn't have during or shortly after the trends of that era were current. The Xbox 360, for example, is over 20 years old and 2 console generations behind. Plenty of time for the people who grew up with it to become nostalgic and have the hindsight to distinguish it from eras afterward, and for younger gaming enthusiasts to gain an appreciation for it as this cool retro thing.