Yes, you can use the software offline. That's what it's designed for. There's no set date for checks. The activation check is sophisticated and flexible. Depends on when it last detected an active internet connection on your machine. On an annual plan, billed monthly you can go up to one calendar month+99 days without one (30 days on a monthly plan, billed monthly).
See also Need to work offline?
It's good practice (but not crucial) to sign in with an internet connection for an activation check just before you leave. That'll give you a 99 day reset.
I signed in today to check my account and I used Lightroom all day yesterday.....so hopefully that will get me by the next 3 weeks.
Can't see any issues with that timeframe.