The USB port can source up to 100mA, as per the USB specification.
Since the available input power is Pin=5V*100mA=500mW, your output current can only be Iout=Pout/Vout=Pin*η/Vout. Typically, η=0.9 for a boost (though it can be lower), so your maximum output current can only be Iout=500mW*0.9/15V=30mA. This is a best-case scenario. In reality, the input voltage will be lower than 5V, due to voltage drops in the USB cable and connectors. Therefore, an output current of about 25mA is a more realistic expectation.