a decade at least. The rating system on a site doesn't always reflect quality, but it does point you toward stories the community has rallied around. Archive of Our Own is my primary haunt now. The tagging and filter system means you can sort by kudos or bookmarks, which is a decent proxy for 'best rated.' The highest-kudo'd fics there are often classics like 'Rise' by wenwen or 'Of Harrowed Hearts'—they've got that staying power. You do have to wade through a lot of newer stuff to find the gems that were posted before the kudos system became the main metric, though.
FanFiction.net is the old-school archive, and its favorites/follows numbers on long-completed epics are staggering. Stories like 'Better Left Unsaid' and 'The Girl from Whirlpool' have numbers in the tens of thousands. The downside is that the search is clunky, and a lot of great authors have migrated to AO3. Still, for that classic, epic-length, pre-2015 feel, you can't beat digging through FFN's top lists.
I'd also recommend checking curated rec lists on Tumblr or Dreamwidth communities—sometimes the best-rated stories aren't the ones with the most algorithm love, but the ones that get passionately recced for specific tropes, like time-travel fix-its or civilian Naruto AUs.