I usually try to transfer the tobacco to a jar the first week of receiving/opening a tin, but I'm not very diligent about it. Right now I've got about 12 opened tins of tobacco that I haven't transferred into jars (really can't be bothered to go buy 12 jars heh, too lazy), the first 6 of which should have reached me about two weeks ago I think.
The one I've smoked the most (i.e. opened and closed the tin quite a lot) is getting pretty dry but is still not too dry to smoke. I really don't want it to get any drier though so I'm definitely going to get a jar for it and pop a coin humidifier in there within a week. Another of the tins I received at the same time but haven't opened very often (only about 4 times) is in better shape. It doesn't feel necessary to pack the pipe and let it dry any but I wouldn't be too bothered if I let it do so for a while, it would still be just fine. I really want to get a jar for it, too, though, before it gets much drier. Need to get my act together and go buy a big batch of jars.
But yeah, in conclusion I would probably avoid not putting it in a jar within at most 2-3 weeks unless you intend to humidify it again.
This all depends on air humidity, temperature, how often you open the tin and how long you leave it open and so on, so others (and you) could have very different experiences.