You really need to do more than just let the beer go flat. You also need to drive off the alcohol which is what will cause a volcano. I've had one lye volcano due to an ale I didn't simmer long enough. That was scary!
The ABV of Guinness is fairly low at about 4.3% (according to ratebeer.com) so it won't take a lot of work. Since the boiling point of ethanol is about 174 F (79 C) you need to simmer your beer at about that temp but don't bring it to a full, rolling boil. I do this by dumping a bottle or 2 into a saucepan and bring it up to temp on a medium heat. Once I'm at 180 F, I maintain that temp for about 15 min. (longer if the ABV is higher). Once it's cooled to room temp I put it in the fridge overnight. You can definitely freeze it if that's your preference.
I've found if I bring the beer to a full boil it scorches the grains a bit so the scent of the soap has a bit of a burned smell to it, which isn't very appealing to my nose.
I also recommend mixing your lye in a container much bigger than you need and putting it into a sink basin. If you do end up with a volcano it'll happen inside the sink instead of all over your countertop, onto the floor and onto your feet. Yep, that happened to me.... Luckily I always wear old tennis shoes when I soap so no foot damage!
