Hi,
agreed, stop watering when the leaves turn yellow.
Spring flowering pleiones: after the leaves have fallen, you can take your pleiones out of their pots and remove all remaining compost gently so that you don't damage the flower bud for the next year. Then you can trim the roots to about an inch. It doesn't matter if they are still a little bit alive, they will soon die. New roots appear in February/March.
For winter flowering pleiones you have to take a little more care. Some of them eg. praecox and maculata, don't seem to die off at all. There it is best to remove what you can of the old compost and just pot them on into fresh compost, taking care that it is damp but not wet and not to water until growth commences in the spring. Keeping them in the old compost is tempting, but it will have turned sour (just sniff it and you will notice). that means the compost has turned acidic and is no longer beneficial for your pleiones. Don't be tempted to put lime on it - it won't work.