I don't know of an average, but I think of it as a process of diminishing returns. When I look at CD's, I evaluate how much more I can get for locking in longer. Right now, A quick look at Fidelity and the 6mo versus the 12mo is 1.9% vs 2.1%. That's not enough for me to go to a year if I'm looking at a short term. The one year to two year is 2.1% to 2.65%. Generally, I might consider that however, in what appears to be a very clear rising interest rate environment, the 1/2 percent for the extra year does not seem reasonable to me. The one year to 5 year is 2.1% to 3.00%. Again, maybe that's enough if I just wanted to set it and forget it, but the raising interest rate environment just doesn't make it reasonable to lock up money for that long for that little difference. On $100,000 the difference is about $900 per year. Not chicken feed, but not that significant.
I guess I'm just not seeing anything that would make me want to lock in a rate for more than one year at this point. I think locking in rates makes more sense when you believe the rates are historically high and coming down. While I'm no predictor of the interest rate market, I don't' think there is any doubt that we are at a low point and going up. FWIW, my gut tells me that historical real interest rates (after inflation) run a couple percent (say 2-4%).