When it was getting popular you'd have a good 100 lines of actual readable text on a screen, with 15 maybe 20 links on the homepage. The content, the draw, was there. Now with their own redesigns, there are ads injected everywhere on the site. On the mobile apps you get 1 ad per 2 thread links.
How can you make the experience 5x worse from when it started and not eventually lose users? Its the equivilent of the makers of a snack food slowly replacing their recipe with filler. One day people will wake up and go, wait this is actually terrible!
So technically "years", but those were the very early days.
There was the whole DVD CSS key nonsense or maybe it was the key for Blu-ray or HD-DVD being censored on the site and the users kinda disobeying the sites mods.
I certainly stopped using the site after that. I never used reddit until recently.
It was HD-DVD
I don't know if spez and kn0thing had some special insight about this, but my experience with it over the long term has been that you can be subbed to a few reddits for your preferred niche interests, and enjoy almost complete isolation from whatever toxicity is going on over in r/politics or wherever else (barring the occasional bit of brigading, which is basically just a mess for the smaller-community mods to clean up).
So there's clearly a large number of mainline users who basically just read whatever reddits are default on the logged-out homepage. But I think there's also a super long tail of users who don't read those at all, and my hunch is that it's that long tail of users who have probably sustained the site over the long term, and will continue to give it a lot of resiliency going forward.