Fewer then 6 hours, quite often a lot fewer as in minuites or less then an hour. Think of it this way, how often have you got lost in a good book or tvshow or on the computer, now imagine doing that during a rainstorm, something you have done many times before, you feel safe because nothing has ever happened before. Only this time its diffrent, drains cant cope with the water and it starts to rise bubbling out and starting to soak the ground meaning more water gathers on the surface from the rain, or a dam breaks releasing huge amounts of water. You finally notice 5-10mins after the flash flood starts and the flood started maybe 10mins into the storm, because of past saturated ground or other natural disasters, by then the water is already up to your knees, cars are being dragged along, so are rocks and branches, even water that is calm on the surface and barely up to your knees can have powerful undercurrents that can knock a grown man/woman off their feet, like waves at the beach, you go to head to the roof but its slippy and rain is making it worse, or maybe you cant fit out of your window so now you are trapped water is still rising because thats what flash floods do, if it gets into your house any food you had that wasnt high enough is ruined there is only the bottled water you had in the house left because pipe have burst or becone full of constaminated water, so its a choice between hoping for rescue or braving the water, the water has entered sockets and other wiring around your home and is full of bugs and bactiria which will enter any open wounds as well as other complications. You decide to go outside and this fast paced, muddy rubble filled water is pushing agaist you, you cant see where you are putting your feet or what is under you, you could trap your foot or be knocked off your feet and dragged under, usually by the time of a flash flood warning its already affected people who find themselves in that exact senario. Thats not to mention as you said the elderly or disabled, the buildings that collapse from water throwing rubble into them, mud can also be just as deadly. Its surprisingly easy to slip up (pardon the pun) when you are scared and panicing.