The mail comes about 2-3 days before departure. I started the process about two days before since the PCR test report had to be ready... it took a while to figure out (didn't upload all required docs first, made a mistake with the booking code and ticket number, etc) but the response comes with 2-4 hours.

On the way to Spain I uploaded my fresh negative PCR test and the Spanish ePLF and was cleared. At the airport the doc checker before the gate scanned my homeprint BP with an ipad and saw that I was precleared.

On the way back I uploaded the german confirmation and the positive PCR test from March and was cleared too. At the checkin desk they asked for all the proof before even entering my name in the system, so the preclearance was of limited use.

The preclearance flags your booking to be 'ADOK' and newly generated (after you get the email) print-at-home or airport printed BPs have the 'ADOK' flag printed on them. The mobile BPs don't have that field yet so the agents checking docs have to scan your BP and verify it in their system.