>> Is it OK to run the vacuum hold down for extended periods of time?
That depends entirely on your vacuum setup. You can make an educated guess as to how long by checking the heat load generated. If your pumps get so hot that they sizzle water, they are likely not heavy duty enough to run hours on end. If you can touch them after they have run for a half hour, they probably are robust enough to last for an eight hour shift.
Vacuum pumps made from vacuum cleaner parts are generally not meant for long duty cycles. Pumps made for industrial use, such as dairy or ponds and such are meant to run all day long and have bearings and heat dissipation methods to keep them from failing.
Another interesting thing to consider is if you have your pumps under your table, and they generate a significant heat load, they can actually change the precision of your table significantly, especially if the heating is uneven. Say you have your pumps at one end of the table and they get to 140f. That means there could be a 70 degree gradient over the length of your table. The change in dimensions of your table would be enough to be measurable.
ShopBot Details:
2013 PRS 96x60x12 (Centroid upgrade)
4hp Spindle
12" indexer
Aspire
Rhino
Fusion 360
Ferrari 360
Prusa MK3S+
Prusa XL multi-tool