I'd say about 30'. Unless you drive a F350 Crew Cab or similar "long truck". But it might be best if you went to an empty parking lot and measured how much "swing" room you need to back into a parking space. Carry a couple buckets with string tied to them to emulate the fence. Move em out till you don't hit them anymore then add 5-10'.
The way I figured was that most road driving lanes are 12'. I can easily back into a driveway with my 18' boat by using both lanes, 24'. I added 6'.