about 80% of the air goes through the engine and 20% bypasses from the fourth stage into the afterburner
A Pratt & Whitney advertisement at
Pratt & Whitney, Aviation Pioneers of Groom Lake - Area 51, Nevada quotes bleed bypass ratio as 20 to 40% (above Mach 2 only). The >Mach 2 is understood, as that's when the bleed is scheduled to open.