I don't think a percentage is the best way to think about it. There is a lot of variability in lifetimes and the time it takes to resolve issues is not related to the lifetime of the cert.

Just pick a discrete number of days before expiration such that you have a comfortable amount of time to deal with issues.

We (Let's Encrypt) recommend renewing with 30 days left, so every 60 days. If your system is automated like we recommend that shouldn't be burdensome, no reason to do it less often than that.