How long can I expect these to last before they need to be replaced?
There really isn't a date these will expire or need replacement as the item is electronic.
Does it repel bats and squirrel from the attic
This item is not made for repelling bats or squirrels.
what does it mean when light flashes very fast?
Green Light-indicated Riddex® is on and should remain a solid green light at all times. Red Light-indicated if the cycle is on or off. The red light will blink for 3 minutes and turn off for 3 minutes. The cycle will continuously repeat as long as Riddex® is plugged in. Blue Light-push the button on top to turn on/off the night light.