I didn't get the chance ot tap anything over the weekend (too busy teaching an axe making course
), but on friday I tapped a tree using the knife-tip method.
Normally I drill an inch wide hole about and inch deep into a large tree and stick a tap into that. The sap seems to gather speed in the middle of the season, but when I did this just over a week ago (so early, one drip every32 seconds or so after the first five mins of 1 per sec) and I got 2.5 litres in 6 hours. The knife tip method - you stab the tree, I made a slit maybe 3/8" wide and 5/8" deep at the point), then knock a twig in that has a flat underside for the sap to run off in to your pot. After the same 6 hours it had produced less than 0.5 litres (dripped just as fast to start with, but slowed quickly and more dramatically).
The auger makes a big hole that needs to be plugged effectively and as rivermoon says, cleanly so that the tree doesn't get an infection or bleed to death. The knife is much simpler and safer (for the tree) but you get far less sap. I've cut a fair number of birch down over the winter and many of them are costantly wet on top at the moment (that's how I knew the sap was rising
), so I reckon the way to get the most sap for a tree would be to cut the thing down and drain off anything you can from the stump before the thing dies.
Only a suggestion if you need to fell the tree anyway!
As for how long you can tap for. People keep saying 2 weeks, but last year I was late tapping and I still got 3 weeks in! This year the sap has bee moving for 2 weeks already and it's not got to full speed yet. So I would think that it very much depends on your location and the amount of rainfall maybe, but at least a couple of weeks and more likely a month