I am wondering if anyone has any knowledge of what the generally-accepted definition of "substantially regular intervals" is in terms of preauthorized EFTs under Reg E - is at least every 60 days as is mentioned in 205.10(a)(1)?

There is no definition in the regulation of the phrase "substantially regular intervals." I'd interpret the phrase using standard dictionary definitions. That would mean to me that it could mean daily, every week, every other month, quarterly, semi-annually, annually ... the list goes on. Using a negative definition, I'd say it's any schedule that isn't irregular.

The "at least once every 60 days" rule only comes into play in deciding whether the notice requirements under § 205.10(a) will apply. They apply, for example, to a monthly social security direct deposit, but not to a quarterly ACH transfer of CD interest from another bank.