A simple silly answer is in the SSD safe stopping distance calculation done in highway design. Here you can get the amount of energy you need to dissipate and the braking time available. If The required braking power exceeds the capacity of your motor, it siMply will not take the load. Instead after absorbing the max power (capacity of motor/ generator) it will keep rotating so long as the total amount of mechanical power is converted to electrical and absorbed by the battery. By then you may have overshot your stopping point.
A 10 Kw generator will be effective braking tool for a bike. The tricks of higher voltage, dynamic voltage transforming , battery capacity will all come later.
A civil engineer says !