Hi, Dan, the 14 days starts running from the point you activate it. So, you could apply it now, leave it there for 6, 12, 24, 48 etc. hours and it'll still run for 14 days once you start it.
Seeing as it's your first one, I know you'll be mad keen to get it up and running asap, I was too, but most of us find that if you apply it and leave it for 12/24 hrs or whatever, it allows it a bit of time to settle in.
You can get bat-sh*t crazy results on the first day or two if you start it as soon as you attach it, because inserting it causes a bit of local disturbance (not surprising seeing as you're poking a needle into your skin) which messes with readings, but it generally settles down after that.
To be honest, you can still get bat-sh*t crazy results with it even after delaying start up. It takes a while to get used to it. It's got quirks. First couple of weeks I used it, I tested way more than I normally did to get a sense of what those quirks where. Once you learn how to read it properly, it's like, yeah, I can see what this is telling me even though it ain't the same as my meter.
Have fun with it!