If you want to detect or image objects at the detection limit of the system
(about 
to 
Magnitude), accurate sky subtraction is the most
important factor. Because of the different thermal background for
different ON/OFF mirror position, one necessarily has to observe the
sky frame by moving the telescope with non changes in the
ON/OFF mirror setting. To avoid large variations in the sky intensity,
one should record sky frames at least every few minutes.
Because one looses some time when moving the telescope,
the effective integration time on the object drops.
The best time per object-sky cycle is therefore always a compromise
between quality of the sky subtraction and the loss in integration
time.
The situation is even worse, if separate observations of
the PSF reference are necessary, because the shape of the
PSF varies faster than the sky emission.