How long should bleach be left in a toilet bowl?

It wouldn't hurt but won't help. The ring is from minerals in the water left behind as the water evaporates. Easiest thing is to deal with the ring using white vinegar when the house is opened back up.

do not do it...... the plastics of the toilet will be yellow when you return.

I wouldn't do it. If she is leaving for the winter, she should drain all the water from the pipes and then put antifreeze meant for RV's into the toilets, etc. As to the mineral build up, there are chemical cleaners that can be found in hardware stores or even in grocery stores that will do the job. Clean the toilet after she gets home after she drains the antifreeze from the toilets and pipes. NEVER mix something that has chlorine in it with something that has ammonia in it. The gas that would be created will kill you. Oh, pumice stone will help remove mineral rings in the toilet bowl.

Yikes! That's a long time. Add a bit of bleach, NOT a cup. Bleach works best at a 1 to 10 ratio, bleach to water. I would say that a a couple of ounces would be enough. A better idea? Turn off the water to the toilet, drain the tank, and with a cup and then towels, remove the water from the bowl and the residual water in the tank. Of course there is water lower (out of sight), but it will get clean, if anything started to grow, over time once the toilet is back on and used.

The bleach can yellow the porcelain permanently. Instead, use car anti-freeze.

Vinegar would be much less caustic or damaging. It prevents mineral build up in a bowl.

William is right.

If the toilets are not going to be flushed for only 4 months, that's not too long a time to have them sit without antifreeze. They will likely have a ring, but it can be scrubbed clean upon return.

Do not forget however, to turn the water off. This will prevent having a flood if any of the pipes or valves crack or break while away.

thats what i do when i go away it just keeps your toilet clean it works good for me no dammahe