What is the minimum ACH debit settlement time needed to reduce fraud risk?

There is no way around it in the ACH system per se. However, some processors do have advanced mechanincs to employ within the ACH transaction framework, for checking up on the account, etc. Based on their internal fraud checks, they can release the funds to you on either a 2-day cycle or a 3-day cycle (typical is almost a 4 day cycle). Under such early release, the processor will charge you a percentage of the total amount being processed for early release of funds (typically hovering between 0.25% to 0.75%) depending whom you work with.

The risk of reversal remains. So best to wait it out for 1-2 additional days.