Depends on what charger you’re using.
2.5 hours sounds about right with the 18-20W charger.
I once set up automations to text me the battery charge level and time (when plugged/unplugged and when it rises above 90%). I got the following with a 120W 4-port charger. The max power draw from the wall was ~39W with just the iPad connected.
iPad Pro 12.9 5th gen
10:35 AM 36%
11:43 AM 91%
12:00 PM 96%
Note that charging is not linear. It goes slower as the battery fills up. I believe the major inflection points are at ~50%, ~80% and ~90%.