Hi,
Probably a stupid question, but since the price of BTC will obviously go down rapidly if a hard fork happes.
Will we get some kind of an annoucement/warning hours/days before they start the fork or will this happen from one second to the next?
Well first of all i don't necessarily agree with your statement. The price of bitcoin will go down rapidly if a hard fork happens, however that is only Bitcoin Core and Bitcoin Unlimited individually. Them added together will probably be the same price as before.
This is what happened with Z Classic, and ZCash, as well as Ethereum and ETC.
As to the warning, we've already got one.
No one really knows when it's going to happen, but if it's going to happen it's probably going to happen this year.
Once the hash power of Bitcoin Unlimited goes over anything over 50%, you have your warning.