we did the borovets to plovidv on sunday 12th Feb and it took exactly 2 hours we left the hotel at 7am and arrived dead on 9am, nice scenic journey down hill, dont know if i would like it up hill, we were diverted to Sofia going so i didnt experience it.
Takes anywhere between two and two and a half hours depending upon snow on the roads etc. This is lateer in the evening, cant comment on afternoon or early arrivals.
Plovdiv airport is basic being an old military establishment, but go prepared (take a little nip for the journey in your hand luggage and time flys!).
Enjoy