They will bounce around until they lose enough energy that they are no longer microwaves, or until they are absorbed by… something. Radiant energy (microwaves, light, heat, etc.) tends to scatter, as it bounces relatively randomly around the room. The problem isn’t that it keeps bouncing, possibly for an indeterminate amount of time. The problem is when its focused, before it gets scattered, or through a microwave element in the oven when its turned on.
Just spending time in the sunlight is likely a lot more dangerous.