Just providing some recent datapoint.

I had a 4h layover at FRA this week. Flight arrived at 9AM, exited terminal and went up to the enrollment office by the Skytrain stop. No line or wait. Went inside and asked about Easypass. The German officer didn't appear to speak much English, she grabbed my passport and gave me a form to fill out. Everything was completed in less than 10min, official barely spoke a word to me. At the end, the official said my status should be activated in around 2h. I didn't try it out since I headed straight back to the gates and lounge. Will try on my next visit.

All in all, very straightforward and easy process.