How long and how often should I soak my beardie for hydration?

When soaking a bearded dragon to increase hydration, I recommend having a Tupperware container that is only used for Phantom (reptiles have the potential to shed Salmonella when they don't feel well, and we don't want to cross contaminate anything in your home). You also want to get a thermometer (a human one will work) to measure the temperature of the water. Ideally it will be 90F for soaking. It can be chin deep, as long as Phantom can hold his head up. if you are doing this for maintenance (i.e. he is not sick), you can soak twice weekly for 15-20 minutes. If you are doing this because Phantom is sick or dehydrated, then I would recommend soaking him twice daily for 15-20 minutes.
I personally don't utilize Pedialyte for soaking bearded dragons, as it can get quite sticky and sometimes they don't like the taste of it. Often they will drink while in their baths, and after that will urinate and defecate.
I hope that helps to answer your question, and good luck!