depends on the shape/structure of it. Runners and small surface areas can be thinner, but large areas like the top of a plenum I wouldnt make thinner than .125" If the surface area of one side is less than 2" you might be ok with thinner. Anything wider than 4" id go thicker, say .1875, and say wider than 6" go .25
I made an intake elbow, the sides out of .125 and then the top out of thinner, like .070-.080 so I could roll it around easier...... needless to say the surface area was more than it could handle and the structural integrity of the thinner stuff flexed a lot and caused it to crack and split. I love tig welding aluminum and am pretty good at it, been doing it for a long time so the welds were not in question. It was just to thin. Its cool to be able to say I blew the welds on my intake though....lol. or not.
These are just my rough guesses. to get good answers you should post a drawing, rough sketch with some measurements.
I dont know man....saying ya blew the welds on your intake kinda sounds to fast and furious................................lololol.......just pcking........ofcourse if I tigged it you would for sure..lmao
Make the entire plenum out of 1/8", runner tubes out of 3/16'" so you have material to blend, and flanges out of 3/8" or 1/2". Making anything in the plenum out of 1/4" is crazy. unless you want to to run 75 lbs of boost.