Falconut,
I have a cabin on the Elkhorn River and another cabin of the Platte River, both in south eastern Nebraska (near Omaha). I am roughly at 41° North Lattitude and have developed a rule of thumb for my specific geographical location regarding when to winterize my water systems here.
I use October 15th and April 15th as the two guideline dates. I drain and store all my water systems on or before October 15th and do not set them back up again until on or after April 15th.
These dates will vary dependant upon your specific latitude, but if you research your local area's weather temp patterns, you will know when to apply such rules. If you have above ground plumbing (pipes and filters etc) then pick dates when the average nightly (LOW) temp begins to hit 34° - 36° F. Somewhere about those dates you should be thinking seriously about disassembling your system and winterizing everything. If your plumbing is below grade (underground) you can wait a while longer, but just remember that you have to work outdoors in the cold, with water, and that can sometimes not be so pleasant if the temp takes a turn for the cold side.
Otherwise, there is no hard rule of thumb to be followed as each pond owner's situation and setup / plumbing varies considerably and moving water takes longer to freeze than idle water.
Catfishnut