All depends on the temps. A couple days in the 60s will do a lot more damage than a week of 20 degree days. But, if you haven't tapped yet, no point in doing it until the sap is gonna run. You could put a couple of test taps out and wait to do the rest until you see some activity. The sap has been running good down here for a couple weeks, but nothing is gonna happen when it's so cold at night and it only warms up for a couple hours.
About vacuum and check valves, yes they greatly extend the possible amount of time you can collect sap.
-Ryan
Went off the deep end. Might be in over my head...