Depends on distance: both Stansted-London , and Heathrow-Reading Railair take about 50 mins to an hour for 50km. I’d tend to see airport buses as express services (i.e., non-stop or limited stop, such as the ones I used to catch 2x a week from Busáras to Dublin Airport, Porte de Maillot-CDG etc). Depending on where you are, or time of day, these may be operated by coaches (luggage underneath) or low-floor buses with extra luggage space). Regular service buses (i.e., ones stopping nearly everywhere) may also serve airports.

From what I recall Busáras was pretty much all long-distance routes plus the airport service. I did go to Drogheda from there, but presumably the bus travelled on to Dundalk or further. Places like Heathrow Central bus station has(had) separate areas for local service buses and long-distance services (including some airport-airport links).

There are also long-distance bus routes which are pretty much regular service buses. For instance Glasgow to Campbeltown takes around 4 hours, but is also the only PT service between towns & villages en route. In Poland, lots of quite small buses seemed to run very long distances (Lublin-Krakow), stopping where needed, which was pretty uncomfortable.

I think the useful rules-of-thumb are : whether a service is turn-up-and-go or whether you can pre-book with a reserved seat (typical of Avanza in Spain); limited stop or not.