In my RPing experience I have had a total of 5 RPs that have lasted longer than a year. I have only ever had success with private 1x1's so I speak specifically of those:
Three of them were extremely slow-paced (sometimes months would pass in between replies) and ended up fizzling out, unfinished.
Two of them were with the same partner. One of which lasted for around two years, spanned several thousands of pages worth of archive content, (it was a massive, ridiculous beastie) and is finished. There was a start, beginning, and an end, conflicts and resolutions, character and plot growth. The second has lasted a little over a year (I believe?) and we are about to wrap that one up as well (it has also reached several thousand pages long.) We have already begun discussing and planning for our 3rd RP together. :D
It's all about finding a good partner who you get along with stylistically and personally, I think. Also important that you're both consistent and dedicated. And having a ballin' plot with engaging characters that you feel highly invested in is also a good place to start.