How much time does an landing/take-off add to an otherwise direct flight?
Let's take the best case scenario to start with, and assume that the cruise ground speed of the airliner is around 400kts.

For our best case, we also assume that the runway is lined up with the flight path and that the airspace is quiet so the plane is cleared "straight in" with no vectoring for separation or holding patterns.


Descent:


A typical descent profile would start 100nm out, during which the plane would slow pretty uniformly from around 400kts to around 140kt. This gives an average groundspeed of 270kt. So this 100nm is covered in around 25 minutes rather the 15 minutes at full cruise.

The descent has cost us only 10 minutes.


Taxi and turnaround:


At a smallish (and well designed) airport the rollout and taxiing to the stand may only take 5 minutes, plus another 5 to attach the air bridge/stairs and open the door. Passenger and baggage turnaround would typically take around 30 minutes, but you have specifically excluded this in your question so we can ignore it. Then getting departure clearances, startup and taxiing to the active: say another 15 minutes.

So this phase has cost more or less 25 minutes.


Take off and climb out:


Again assuming we're cleared for immediate take off, it takes around 20 minutes to climb to cruise altitude, at an average ground speed of (say) 200 knots. This means we've covered about 70 miles, which the cruising aircraft would have covered in just over 10 minutes.

So this final phase has cost us another 10 minutes.

Total for the best case scenario is 45 minutes.

Of course the best case is not that likely. If the airspace is busy, holding for another 10 minutes before final approach, and 10 minutes further wait for departure and takeoff clearances, would not be surprising. And if the active runway is in the "wrong" direction or vectoring is needed for traffic or noise reasons, that can easily add another 10 minutes or so.

So in the non-best case scenario, a total loss of 1h15 to 1h30 would not be unreasonable.