How often do annuity rates usually fluxuate?

Assuming that "annuity rate" means the rate of interest paid on a deferred annuity, the answer depends upon whether the annuity is a VARIABLE one or not. The contract value of a VARIABLE deferred annuity is tied to the investment performance of the separate accounts chosen by the purchaser. These accounts are much like mutual funds, and their value will fluctuate, often daily. In a VARIABLE deferred annuity, there is no guarantee of principal or minimum interest (unless you've invested in the "fixed" account).

In a non-variable annuity, often called a FIXED annuity, principal, and a minimum rate of interest is guaranteed. In addition to that minimum rate of return, most deferred annuities offer additional, non-guaranteed, interest. There are two types of fixed annuities: (a) "Declared Rate" and (b) "Indexed".

Declared rate deferred annuities generally declare the current, non-guaranteed interest rate each year. Index annuities may declare and credit interest each year or retroactively, after several years.

For more information, see Olsen & Kitces,"The Advisor's Guide to Annuities" (3rd ed.,2012, National Underwriter Co.) or "Olsen & Marrion, "Index Annuities: A Suitable Approach" (Olsen & Marrion LLC, 2011).