This itinerary explores the popular Jungfrau Region near Interlaken. I think 3-5 days is the perfect amount of time in the Jungfrau Region. If you stay 7-10 days, I would add days from my Bernese Oberland itinerary.
I've included at least one day on each mountain area in the Jungfrau Region.
Grindelwald First
Mürren-Schilthorn
Schynige Platte
Männlichen
Jungfraujoch
Pfingstegg
Lauterbrunnen Valley
I include options for different kinds of travelers: hikers, non-hikers, and families with kids/teens. Some days include a lot of activities. If you want a more relaxed pace, spread the activities over multiple days.
Please note the closing times and organize your itinerary carefully so you aren't stressed rushing from one thing to another.