As always you want to look to the datasheet of the specific part, if available, for the standard operation and maximum ratings. Then you can use that information to determine the current through the LEDs you want to set using the resistors. First total the voltage drop of the LEDs (if you are using more than one) and subtract from 5 VDC, that's the voltage drop that will be across the current limiting resistor. Then use Ohm's Law, VR = IR * R, to figure the resistance that will get you closest to the desired current.
Of course if you are powering the LED directly from the Arduino's I/O pins, this current should always be below 40 mA. I'd suggest keeping it below 35 mA or even 30 mA if the LED will be on for prolonged periods to be more certain you won't damage the I/O pin.