How long is the Pilgrim's Mass in Santiago?

I think it lasted about an hour, I seem to recall it finished by midday.

There is a bus stop in Plaza de Galicia for the airport, about a 10-15min walk. Journey time was about thirty minutes.

It lasts roughly an hour and starts at midday. More information here.

If you want to be sure, there are usually taxis in the main Cathedral square, outside of the Parador or in the adjoining side street, the Rua de San Francisco. I believe there is a set fare of around EUR 20 to the airport and you should easily do it in under 1/2 hour (it is approx 15km away), leaving you two hours before your flight .