I usually stretch it out for as long as I can. I don't like reaching the "end" of the games, so I tend to procrastinate. (This has lessened somewhat since I sat down and wrote up my backlog last summer, as I'm feeling more pressure to actually finish my games now, but so far, that hasn't really translated into finishing Pokemon games any faster. If anything, I've been playing non-Pokemon games to balance out all of the Gold I've been playing before bed, rather than finishing my gen 7 games, which I haven't beaten yet.)
My faster games tend to involve beating the Elite Four in a period of a couple of months. For slower games, I think my longest time to reach the Elite Four was approaching a decade (in Pokemon LeafGreen, after 323 hours or something like that). Admittedly, for the slower games, they're not the only thing I play for that duration. But I usually have a fairly high hour count and reasonably complete Pokedex by the time I "beat" the game for the first time.