It seems totally inconceivable to me that you could accurately predict how long until we will know whether BB(7) is greater than Graham's Number.

I do not predict that. We just need the bet to have a time limit because BB(7) will always have holdouts as long as I live. I chose 10 years because I have prior experience with that timeframe [1].

[1] https://senseis.xmp.net/?ShodanGoBet