Those sandbags are meant for one time emergency use in actual floods, not for keeping normally planted things from root rot. Your plants should be in well drained soil to begin with, so even if there is an actual flood, they have a better chance of survival. Were you seriously thinking of digging trenches near your plants to put sandbags in? Won't work - the ground is saturated from below to a great extent anyway, apart from the area immediately above a given plant, so there really isn't a lot you can do unfortunately.

How long your plants can withstand being under water depends entirely on the variety of plants. Some can withstand better than others.