It varies, you just have to keep checking your tree's. Having said that, with the lack of a cold winter this time around, I'm seeing plenty of signs of leaves starting to bud etc but haven't bothered to do a proper check yet, indeed I probably won't bother at all because I've no use for the sap anyway.
I was visiting a local woodland last spring (April 20th in fact) and the lad who was the tenant, was clearing pathways for visitors by the simple method of chopping off small branches with a machete. This was the result;
[video=youtube;YLbHcnRWDUE]https://www.youtube.com/watch?v=YLbHcnRWDUE[/video]
You can check the flow, by simply pushing a blade tip into the bark of the tree, if you get drops of "water" the sap is flowing. So get out there and check your tree's.
As for "dangers", well........do you own the tree's you intend to tap? if not you're liable to have issues with the people who do own them but not very likely unless they are growing in someones garden. The only other problems I can think of at the moment would be from falling dead branches and bacterial infection or "germs" from the trunk of the tree.
Do a search on here and I'm sure you'll find plenty of relevant info.
atb
Steve