As soon as you put money in there. The sooner you start, the more you can earn!
Keep in mind that interest is paid in whole cents only after rounding. At the end of each interest payment period (usually monthly), fractional interest equal to half a penny or more will be rounded up and amounts less than half of a penny will be rounded down. If your balance has accrued less than a penny of interest at the end of the month (after rounding), you won’t receive an interest payment.