Distance from Ardabil city to Tehran is 591 Km.
Almost is 6 hours to travel by car.
But the airport "Mehrabad" Tehran to Ardabil city done daily flights, flight duration is less than one hour.
For a road trip by bus from south and west Terminal can use "Ta'avoni" they are mentioned below.

Name Telephone
Ta'avoni 14 02144662853-02144663867
Ta'avoni 4
02144654651-02144630005-02144666030
Ta'avoni Iran Peyman
02144662745-02144657638-02144657324
Ta'avoni Seyro Safar
02144690501 
Ta'avoni 3
02155185333-02155185330-02155185329
Ta'avoni 14 02155185393-02155185394-02155185397