There is no certain time for this, as long as your coolant temperature is ok. It could be wise though to change it when you have the cooling system open for an other reason (ie waterpump or radiator change for example).
JEEP Grand Cherokee 4.7 Lmtd 2004
HID 8000K in low and fog / Quad Squad member
KYB Gas-a-Just shocks / Pirelli Scorpion ATR 245/65/17 - Transgo reprogramming shiftkit.