How much power will a 750 watt motor take at full power? 750 watts would seem to be the answer, though in reality such a motor can probably handle a lot more than that for a short period. (The power ratings are usually largely about cooling.) Though I really don't understand what you're asking.

But as for recharging your battery to extend your range ... you want numbers?

Suppose your electric bike will use 250 watts to propel you at 20 mph on level ground.

You then add a generator that is powered by the wheels (or a turbine powered by the air, whatever) and this generates 40 watts of power that you add back to the battery.

However, you now find that your bike needs 350 watts to go at 20 mph on level ground. So you've spent 100 extra watts to generate 40 watts, reducing your range.

As for what regenerative braking (not breaking) is, consider that it's off most of the time. But if you want to slow down, rather than hit your usual brakes, you hit the regenerative brake, which slows the bike down -- by using a generator to charge the battery. So you recover some of the energy as you slow down, and that does extend your range. And since motors and generators are the same thing, in general you don't even have a separate generator at all -- it's done with the same motor you use to power the bike in the first place.