So eventually I need to replace my 5S. How long do you think I have before 7.1 comes out and the replacements in the stores have 7.1?
Nobody outside of Apple can answer the first question. As to the second, I've seen replacements keep old versions of the software for weeks or even a month or two after an update has been released. It's a gamble though so it's probably best to get it replaced sooner rather than later if a jailbreak is really important to you.