You'll know quickly if you've been hit, as you can't open the affected files. A delay wouldn't change anything. And the bad guys have no reason to delay the encryption process. We've had several clients tell us initially that there was a delay, but it turns out they just didn't notice the damage for a day. The dates/times showed the process takes hours, even perhaps a day if the number of files is great and the network connections are slow. In one case, there was a large USB drive attached to a workstation that contained tons of jpg files. Because the infection usually encrypts the infected computer first, it took hours to encrypt all the files on the external drive. Luckily, when it finally starting encrypting files on the server, it was caught right away, and the encryption only affected files starting with the letter "A". The ones I've seen work alphabetically.
But it's always good to have an "Off-line" backup. I've seen several backup formats, notably image files, that don't seem to be get encrypted. The newest twist, though, is encrypting the whole drive, rather than the files.
