FIrst, be a doubting Thomas and never trust supplier propaganda sheets, do your own testing. How often do you knead the paste? We have a standing requirement that if the paste printer is idle for 30 minutes, we have a minimum of 4 kneads before printing.
To test for stencil life in a recent solder paste evaluation, we let the paste sit idle on the screen for 0.5 and 1 hour. At those time intervals, 5 boards were printed and we checked solder volumes on the pads. All of the pastes needed at least 3 boards to get the volumes back up.
In the past we ran a study with paste left on the screen overnight, and with at least 5 kneads, we had no problems. Manufacturing didn't buy in to it especailly during the summer months (I'm in the sunny south with normal outside humidities of 90 - 95%). Some of the folks around here said in their previous jobs it was common practice to scrape off the stencil, put the paste in a jar, then mix it with new paste the next day.
Again, what works for us won't necessarily work for you. Set up a test matrix, execute it, and see what your results show you.
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