Here’s the most recent T Bill rates: https://www.treasurydirect.gov/auctions/announcements-data-results/
The last 26 week rate was 5.381%
SPAXX 7 day yield is 4.75%
T Bills are also state tax exempt, CA 9.3% bracket, which makes the T Bills more like 5.88%
To be honest, the 1% extra is probably only yielding me around $500/yr, but I have learned a lot more about fixed income over the last year than I've learned while rates were low.
I've never been in the position to have to decide when/if to extend the duration of my bonds, hence the original question. My gut is saying that sometime this year I should extend to around 2 years duration and that by next year rates might start going down. But of course I don't know nothing!