This is never as long as they advertise (so a 2.5 hour flight is often in the air for less than 2 hours
Apologies if you know this but the official sector time on timetables and the official times of departure/arrival you see on most websites etc are based on times leaving the gate>arriving at the gate...
Flightradar often displays times differently for reasons that are maybe for another time.
So unless you're unlucky and are on a longish flight that ends up plodding into very unfavourable headwinds you almost always find your actual flying/airborne time is less than the sector time. ..funnily enough the difference is usually around 30 minutes, exactly as you mention.