Reading it can take days or hours to transfer it in to fiat again.
The bitcoin transaction could take up to several hours/days when there is a spike and/or your fee is too low.
However, with a good wallet (e.g. electrum) you will be able to bump your fee. This can ensure that your transaction will get confirmed faster (for a higher fee, of course).
The conversion into fiat itself doesn't take much time.
During that transaction the value of the crypto changes. I assume and hope that it won’t effect the transaction in anyway? The price you sell is fixed I mean regardless the action that’s going on in the mean time.
The BTC/USD rate doesn't matter when you are transacting BTC. 1 bitcoin will always be 1 bitcoin.