I think the general rule is that the silicone should be 1/4 inches thick, but this can also depend on how many copies you need to cast from your mold. Are you planning on pulling 1 cast or 20? If you only need to make a handful of casts, a thinner silicone jacket should hold up fine...If you want the mold to have some longevity, then anywhere from 1/4 to 1/2.
I can't chime in on the plaster jacket since I've only ever used Plasti-Paste for my molds.
as a rule i try and get a wall thickness of at least 10mm that way the chance of distortion is reduced. i have had molds that were 5mm and sometimes under and it always produced a poor quality product. i use a fiberglass mothermold for everything also. keeps the weight down.
This is based on Rebound 25 from Smooth-On. Different rubbers will be different, but I would recommend 1cm or 3/8in thick. For a mold that will just sit there you might be able to make it thinner, but if you are rotocasting you might want to go even thicker. The first mold I made for rotocasting was too thin and and the cast part looked like it had been in a hail storm. And of course the master got destroyed when I made the mold.