Personally I like the 10% rule, maybe a bit more aggressive than 10%. I think it also depends how well you want to feel at the finish line.

My longer weekend rides are 30 to 35 miles on Saturdays and 45 miles on Sundays. This has been my schedule for months and the 45 miles ride is really no big deal.

I recently extended my 35 miles to 66 one Saturday since I was feeling good that day. Yes, I made my goal of the day but it was not fun at all to be honest. Environment was definitely a factor as it was particularly hot that day, and I probably should have left earlier in the morning than I did.

But as I gear up to get towards the 100 mile mark, I'll be adding miles on to my long days slowly, 5 to 10 miles at a time.

So, in my very humble opinion, and admitted lack of experience, I would say one is ready when they can comfortably do 75 to 80℅ of goal.