In general, the public transport in the Parisian region is fairly reliable, effective and efficient, despite the dirt and grime. However, exceptions apply. The last 2 days of public transport strikes have been a nightmare.
My personal record to travel 8km by bus to work: 1.5 hours.
(Usually 15 minutes by train)
Pépé’s personal record to travel 16km from work to home: 2 hours.