When missus 9A needs a wheelchair to the gate, which is not always, we arrive at the airport 3 hours before the flight. Out of about 25 flights together with her in a wheelchair we have needed every minute of the 3 hours three times (EWR, SFO, LHR).

Either the skycap at the curb or any check in agent can call for the wheelchair. There is an area at EWR and SFO to go to to wait for it.

At EWR it can take 45 minutes or more for an attendant to be free and (IME) they do not let you push someone in one of their chairs without an attendant. At SFO I can often find an un-staffed wheelchair in baggage claim and once we are using it, no one minds.

Going through security takes longer and depends on whether the person in a wheelchair can walk through the metal detector. If they can walk then they walk while TSA screens the wheelchair. That takes an extra pair of TSA hands so can often add to the time.

At boarding they will call for wheelchairs first and passengers traveling with the wheelchair rider can go down together. If you can walk from the end of the jetway to your seat, all is good, if not you are probably in your own chair, but they will get an 'aisle chair' to transfer you to and bring you onto the plane. On the other end the process is reversed. If you have wheelchair assistance in your record, they will be waiting with a sign with your name at the end of the jetway (usually) and can take you all the way to baggage claim and the curb.