Also, consider limiting annuity to things immune to inflation (ie; mortgage or similar fixed debt / payment).
I am a recipient of a $7,000 annuity like "buyout" that initiated in 1992. Today, that same buying power would require over $12,000. And that only took average inflation rate of ~2.5%. This buyout expires in 2021.
I've had a very lucky run. It also taught me to be leery of annuities in any form.
A 5 or 10 year span of higher interest would just crater the buying power of an annuity. There a plenty of inflation calculators on-line. You should model multiple scenarios. In example 1960-1990, inflation was 5%. Yikes!!