As long as possible if it's a great game.
For example Xenoblade 2 took me about 70 hours for the main story, but I continued playing for another 300 hours.
What's conspicuous imo is that jrpgs nowdays are much longer than in the past, but they often feel like they *should* be even longer. Take Xenoblade 2. 70 hours is enormous, but the game actually is missing proper character arcs for: Moragh, Poppi and Dromarch. Also the Zeke-Pandy relationship is super strongly implied but we never get a scene that truly resolves it. If these parts were included, we'd be looking at at least another 10-15 hours!
And then I remember when I olayed Skies of Arcadia Legends on the GameCube. And how was 'proud' when I saw the game clock say '40 hours'. Final time is a bit higher, but definitely below 50. And that used to count as 'epic adventure'. Nowadays a 50 hour jrpg is considered short one, with all popular franchises going all the way up to 100 hours!
Anyway, if I have a good experience, I don't want it to end. Especially jrpgs, where you grow close to characters and their world. Makes me want to fire up Xenoblade 2 again ...