USB drive. If you lose that but you still have the wallet on your PC what do you do, Do you need to run to your PC as soon as poss and transfer the funds?...
Yes. That's exactly what you do.
If you have send most of your bitcoins to a wallet you've stored somewhere offline.. How long can that wallet stay offline before the money is lost?
Forever. There is nothing you need to do to receive or hold bitcoins other than having the corresponding private key for the address stored somewhere safe.
what happens if you've encrypted that wallet and you forget the password?
Assuming you have no accessible copy of the private key(s), the coins are lost.