When do you go big on 10, 20, or 30 year bond?

At what rate would you go big on the 10, 20, or 30 year bond and lock in your earnings? People do this for mortgages. Why not have a steady income stream for 10, 20, 30 years? I'm talking about holding until maturity and not speculating. Is it based on how much money you have? Perhaps 5% rate is great if you have 10 million bucks. That's 500K per year to live on.

I don't think inflation will come down to 2% anytime soon. China's rise caused deflation and kept prices in check. Those days are gone. I'm betting long bonds will go much higher but want to know what criteria to lock in and forget about it. I also think market average return for the last 40 years is a bit irrelevant. Rates went down for the last 40 years to zero. That's a huge tailwind that's not going to happen again for decades.