You don't even need to leave it in there for 30 minutes. Generally a chin will be done rolling after the first 10 or 15 minutes tops. After that, the dust bath becomes a litter box. It's a huge waste of dust and it's a mess to clean out (especially when they urinate).
You can prolong the life of your dust. There is a fine grain scoop sifter that is sold for reptiles that is pretty handy at cleaning out the mess. You use it just like you would a kitty litter scooper. It cleans the dust out nicely for re-use. When it starts to get too grainy, you just throw it away.