PWM duty cycle vs candelas for LEDs
You can assume that duty cycle and amount of light emitted are linearly dependent. Usually the rise/fall times of the PWM signal and the LED's brightness are very small compared to the "on" and "off" times so that one may approximate them with an ideal rectangular wave form.

As John U said, however, in reality this may depend on the quality of your PWM signal, the impedances of power lines, etc.

Besides, one may note that many LEDs can be driven at a higher-than-normal current when used in pulsed operation. This can be used to get, for instance, 10% of the light output with 5% duty cycle. (Which then again may seem like 20% of the brightness to the human eye.) The limits for this kind of pulsed operation are usually specified in the datasheet of the LED.