If I pedaled my bike generator at 75 watts an hour how long would it take to recharge a 12v car battery.?

Generating 75 watts of power (at 12 V) produces 75 W 梅 12 V = 6.25 amps.

If you pedaled for one hour, you would have put 6.25 amp-hours back into the battery.

Your success in recharging the battery depends on how far it was drained (discharged) ahead of time.

If you're using this battery for something other than operating a vehicle, then you probably want to use a deep-cycle battery. These are intended to be repetitively charged and discharged. They also have a higher amp-hour capacity for low drain equipment.

You to have to tell us how many amperehours is the battery.

Large deep cycle batteries used in small fishing boats are usually rated somewhere around 100 amp-hours. Using the 6.25 Amps calculated in an answer above, theoretically 16 hours. However, typically battery charging requires approximately a third more amp-hours of charging than what is actually used.