How long will brakes work after Hybrid (AM-2) fuse is blown?

indefinitely.

 

there is a mechanical connection in the brake system, so you always have brakes, albeit, old fashioned non power.

 

Why not go to an empty parking lot (perhaps with a slope) and disconnect your 12v and unplug all the harnesses from the black box next to the 12v and see if there is any mechanical link? You can always use the parking brake if it doesn't work.
While in there pull the orange hybrid plug..
Either the brakes will work or not, there's no in between in that scenario

Edit: give me ten minutes, my car is stripped down in the back and my driveway is sloped. Stand by

 

Ok there is definitely a mechanical link. I put my car in ready and put the parking brake on, pulled my 12v and hybrid plug and the backup brake box harness and was able to roll down my driveway (confirming the car was not in P) and I pumped and stopped about 50 times before I got to the end of my driveway. There is absolutely a mechanical link.
Edit: I clearly forgot to list a step for anyone wishing to duplicate this: put the car in N before pulling the 12v..

 

No problem.. I test hv packs on the road and I regularly stop at rest areas and swap packs super quick before anyone says anything, so my car was ready for this one my guess is the backup system is just to give a little comfort to people who expect the brakes to behave as normal in the event of a catastrophic systems failure. While I had to press a little harder than normal, it was not even remotely hard to jerk the car to an instant stop (although at maybe 3mph max) and I have no doubt it would work well at higher speeds.

 

Yes, the capacitors extend the power assist for a while in the event of loss of external power to the braking system.

But federal passenger car safety regulations also set standards about how the brakes must perform in the event of nearly any single imaginable power assist failure, short of bursting a housing that drives two hydraulic circuits. This pretty much requires that a mechanical (hydraulic) braking system underlie any power assist system, still functioning (albeit with a lot more pedal force) when the power layer quits. For the Prius' top level brake-by-wire system, this means the driver must be able to stomp through it to reach the raw mechanical-hydraulic system.

Those federal regulations require that with power assist completely disabled, and some other conditions (e.g. a certain amount of wear on the friction parts) the car must stop from a certain speed (100 kmh -- 62 mph?) in a certain distance (considerably longer than we are accustomed to, I think something over 160 meters) when a certain pedal force (500 newtons, or 112 pound-force) is applied.

Somewhere in the past, I posted a link to the actual regulation. Now that 'Search' is claimed to be working again, maybe I'll even be able to find it.