The usual time is around 20 seconds, but with already a clean mouth and with consistent preventative care, it is not an issue. However, with already diagnosed periodontal disease, that is a concern for the toxins in the area will start the eat away at the bones around the teeth, causing more problems. It is a cycle that needs to be maintained for optimal health. Plus, that also means other systemic issues may be at risk, too.
CORRECT FLOSSING AND BRUSHING MUST BE DONE EVERY DAY.
Some bacteria have the ability to replicate every 20 minutes (see how many you would get after 24 hours). So, if you brush your teeth one night there is a layer called the salivary pellicle that forms on your teeth almost instantly. After this adheres, the biofilm that we call plaque starts forming and the bacteria started growing and replicating. If you sleep for 8 hours and you started with 1 bacteria and it replicated every 20 minutes you would have 16,777,216 bacteria (and that is if you only left one behind when you brushed your teeth). So, plaque starts forming and builds up after a few hours.
Hope this helps.
My best to you!
William F. Scott IV, DMD