It depends on the system. Also it depends on how often you
should clean it and how often you
must clean it. In the growing season as fish are being fed, it is best to flush every day. This is better for your water quality and better for you fish. In some systems it is also needed to keep the screens from clogging and failing to work. Other systems are less critical and can go for weeks in an emergency.
Something like the Answer is mounted in a vortex and the detritus sinks to the bottom of the vortex which acts as a sump. New water tends to pass over the crap and is passed on to the next chamber. The down side is the increased use of power for the pump that keeps the screens clean.
Something like the laser-cut parabolic screens (EA, Estrad, etc.) need no extra pump but as the crap builds up a portion of the water passes through it at the bottom of the screen. Even though you don't need a 2nd pump bear in mind that you will need extra pump head to raise the water 2'-3' from the bottom of the sieve up to the level of the filter.
There are a number of DIY flat screen sieves that will work quite well and cost little to build but these need constant attention. Failure to clean them means they stop working.
Do remember that all these system not only remove the stuff that settles on the bottom, but also remove suspended gunk as well. Bits of algae, leaves grass, in short anything that doesn't sink to the bottom but gets sucked into the filter drain.
The down side I see of flushing from the bottom as I seem to understand you plan to do, is that you need to remove an awful lot of water to get the crap sucked in from any distance from the drain. Sure the stuff will start moving but you sump will be full before most of it finally gets to the drain and into the sump. You then need to allow it to settle after which you will need to pump the water back
I have a couple of mid-level drains in my pond as well. When building it I was under the impression that I needed to leave the water on the bottom alone in the winter so a "warm layer" would form. Utter twaddle as I see it now. When I change/deepen my pond next time these will be the first things that go. If you ask me the best and
only place to remove water is at the bottom.
Bead filters... I don't like them and would never trust them to get the job done. OK for polishing water but that is all, I would never use them for biofiltration.
I flush water from the vortex before the Answer filter. A laser-cut screen would have no trouble with you flushing through them though. In general I will close off the filter system, isolate the vortex and allow the entire vortex to drain dry. I then open the valve to the bottom drain and allow the water to rush in one go. This cleans out the pipes and sucks up and crap near the drain. I dump the vortex a second time and allow it to refill with the now clear pond water. I do this every other day in the summer and once a week in the winter. In-between it is enough to pull the drain valve from the vortex open for 15 seconds to allow the settled crap to be washed to waste.
I hope that about covers it. If you need more specifics please ask.
B.Scott