Still making moonshine?!

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To simply answer your question, you do not want to evaporate all of the mash because it is probably only 5-10% alcohol (and not possibly above 18%), the rest is water. It is best to monitor the temperature of the vapors before condensation, once the temperature reaches close to 100 C most of the distillate will be water, at which point the mash can be discarded. The process can be repeated with the distillate in order to achieve higher purity. Using a thermometer is also important because you do not want to drink any of the distillate that is produced before at least 78 C because it is probably harsh aroma compounds and most importantly methyl alcohol, the stuff that makes you go blind.

But it depends on the type of still you are using. My personal "water purifier" does boil the wash, but I monitor the temperature at the point where the vapors travel into the condenser after a long trip through a surface area rich column. This allows me to separate the dangerous heads by discarding all distillate produced before the temperature reaches 80 C.

FYI a good makeshift pot still is a crock pot because it heats up slowly.