Piper that depends on what voltage and what LED you have in there - generally i use a 470 ohm for 4.2v straight through - it's a little overkill but the LED's are bright enough and not had on burn out yet
What is the rating of the led and what power are you putting through it? Check this out - http://led.linear1.org/1led.wiz
Some LEDs come ready to fire at 5v all ok so a 3.7v mod will usually be fine and just light a little dimmer.