Jajce is a compact town with a clear set of highlights. Unlike cities that reveal themselves slowly over several days, Jajce is readable quickly. The question is not really how many days you need -- it is whether the stop is a transit break, a half-day, or an overnight.
Half a day: the most common approach
Four hours in Jajce covers everything essential: the Pliva waterfall, the climb to the fortress, a walk through the old town, and a stop at the Pliva Lakes and watermills if transport is available (they are about 3 kilometers outside town -- a taxi costs roughly 5 to 7 EUR return). This is the right structure when Jajce is a route stop between Sarajevo and Split or between Sarajevo and Banja Luka.
Most travelers passing through on a bus route can arrange a stop of a few hours between connections. The main bus station is centrally located and everything is within easy walking distance.
One overnight: when it adds value
A single overnight in Jajce is worth considering in two situations. First, if you want to see the waterfall in the early morning light before any visitors arrive -- the experience is genuinely better than at midday. Second, if the route naturally breaks here and the alternative is a very long transit day.
Guesthouses in Jajce cost 30 to 50 EUR per night for a double room. The town has a handful of good simple restaurants serving Bosnian food at prices well below Sarajevo or Mostar. The evening in Jajce is quiet -- this is a small Bosnian town, not a destination with evening activities -- but that can be exactly what the trip needs after several busier stops.
Two nights or more: not recommended for most travelers
Two nights in Jajce is more than most travelers need. After the waterfall, fortress, old town, and lakes are covered, the town does not have the depth to sustain a longer stay for most visitors. The exception would be travelers who specifically want a very slow, quiet Bosnian small-town experience as a deliberate counterpoint to the busier stops in their itinerary.
Practical transit notes
Buses between Sarajevo and Jajce run several times daily and take about 2.5 to 3 hours (roughly 8 to 12 EUR). Buses toward Split from Jajce involve a change at Banja Luka or go via Mostar. Check timetables carefully before building Jajce into a tight connection -- the small-town schedule is less frequent than major city routes.