Well, it all depends on location, and your SWR's
if you have checked and tuned your antenna, to have SWR's below 1.5:1 then good, anything above that can majorly impact your effectiveness, Also if depends on where the antenna is. if its on a bumper or somewhere lower than the top of the car for more than 2/3rds of it, it wont do very well, if its mounted on one side of the vehicle you will be VERY directional, so one way youll talk and hear alot further than another way (unfortunatlly its generally to the side if its on the side of a vehicle)
But! lets say the antenna is set up real nice, It then comes down to the geography of the area. if its real flat, not many obstructions, you can talk pretty far, if its city like and lots of mountains, you may not talk as far, but as a rule of thumb CB 's are thought of to talk 5 miles. If everything is set up real nice you will probably talk 10 miles here and there.
sometimes youll talk even further. CB radio is fun, and thats one of the great things about it. Keep doing research about antennas, and eventually youll be talking with the big dogs. My self in my truck, i have talked over 20 miles to another vehicle, this is pretty average for me, but ive got a nice set up. on SSB ive talked much further with out skip.