That kind of question is the kind of thing you should ask the club, since it may vary club to club.

Generally, they are paid at the same time, at least they are at my club. The reason for the one-time fee is usually to go in to an account to pay for field improvements or save for a new field. Often times (as at our club) people who were members at some time in the past were assessed a set amount for a one-time big expense of building a field. To keep everyone from quiting the club, then re-joining later, after the fee "expired", it was made into a perminant one-time fee for all new members.