One night is often enough for the right trip
Jajce is one of the more efficient scenic stops in the region, so one night can already work well if your route is moving through Bosnia and the goal is atmosphere over maximum sightseeing. The waterfall, old core, and one good evening are enough to make the stop feel worthwhile if the stay is set up properly.
That makes it easier to add than many travelers expect.
Two nights is better if you want the scenic side too
Two nights becomes the better answer if you want Jajce to feel more complete. The extra time helps if you also want the fortress views, the Pliva Lakes side, and a slower morning rather than just a fast arrival and departure. Jajce is still compact, but the second night gives it room to feel more like a real stay than a scenic interruption.
For couples and slower travelers, that extra time is often valuable.
Who should not overstay it
For most travelers, Jajce does not need a long commitment. If the route already includes bigger Bosnia bases, one or two nights is usually the correct range. The town is memorable, but it is memorable because it is concentrated. Stretching it too far can dilute that advantage.
It works best when it stays sharp.
The best first-time answer
If you are unsure, start with one night if Jajce is a stop inside a wider itinerary and two nights if you want to enjoy both the town and the lakeside side properly. That is usually enough time for the destination to feel distinct without overcomplicating the trip.