We use a single deek just to give the ducks a bit of extra confidence but we leave it there all the time. This probably isnt the best way to do it, but as we only shoot the pond as the finale in a walked up day, we would never remember to bring the deeks with us if they were removed each time. Good advice above, but the long and the short of it is, experiment, find out what works best on your pond and with your birds.
Best of luck,
TT.