The characteristic of LEDs can vary quite a bit. The voltage drop usually depends on the color, but your LED might be different. This is just a guide, check your datasheet or meassure it. Typically red, orange, yellow LEDs have a voltage drop of 1.8V-2.0V and green, blue, pink/purple, white of 3.2V-3.3V.

red orange yellow					green blue pink/purple white
V/mA	5mA	10mA	15mA	20mA	V/mA	5mA	10mA	15mA	20mA
3.0V	220Ω	100Ω	82Ω	68Ω	3.0V	-	-	-	-
3.3V	330Ω	150Ω	100Ω	68Ω	3.3V	-	-	-	-
3.7V	390Ω	180Ω	120Ω	100Ω	3.7V	100Ω	47Ω	33Ω	33Ω
5.0V	680Ω	330Ω	220Ω	150Ω	5.0V	390Ω	220Ω	120Ω	100Ω

red orange yellow					green blue pink/purple white
V/mA	5mA	10mA	15mA	20mA	V/mA	5mA	10mA	15mA	20mA
3.0V	100Ω	47Ω	39Ω	33Ω	3.0V	-	-	-	-
3.3V	150Ω	68Ω	47Ω	39Ω	3.3V	-	-	-	-
3.7V	180Ω	100Ω	68Ω	47Ω	3.7V	47Ω	33Ω	22Ω	15Ω
5.0V	330v	150Ω	100Ω	82Ω	5.0V	180Ω	100Ω	68Ω	47Ω