I'd say around 5 minutes, I usually pump up the pO2 a bit and end the prebreathe once my solenoid fires on <0,2 pO2.
My take away is that I need to focus on the unit even out of the water (a bad habit I have from DM-ing on open circuit where I would check my stuff as I was jumping off the boat, that's not gonna fly with the death machine I seem to have bought). I'm 99% sure I would notice the solenoid not firing in the water even if I was not checking my pO2.
Got it, so you just drew the short straw and had a solenoid failure after a good prebreathe! So glad you were aware enough to "bail out" on the beach. Thanks for sharing that story.
It's off topic for this thread, but I do like discussing CCR failure modes - I gave a talk on checklists for one of Gareth Lock's seminars and I learned that there is a lot of room for improvement in our little bubble-free world.
I remember a similar issue on a boat dive where someone (who didn't do a long pre-breathe) stood up, walked in gear from his bench seat to the transom and collapsed on the deck before he made it. Most dive boat crews that I see now ask you to give them your PO2 before they let you splash.
I can't ever hear my solenoid.