Your word refresh will happen on the date that you subscribed to the plan, or if it falls on the last day of the month, the last day of each month. Unused words do not roll over for monthly plans, but will roll over from month to month on annual plans.
For example, in the Individual I plan, if you subscribed on the 31st of March and you've used up 18,000 out of 20,000 words by 30th April, then your account will be reset to 20,000 words on 30th April.
For annual plans, all words will be given at the beginning of the subscription.