Honestly it's probably agent dependent. I've through checked on separate reservations that are short and long layovers, and I checked through a 16 hour LHR layover last year; the bags just went into storage in the system and were pulled out a few hours before the flight (always safe to be asking the agent at the airport to check on it, though). It's a manual process for through-checking, though, so some agents may not know or want to do it. That being said, under 24 hours it shouldn't be a problem. Over 24 hours, I'd want my bag anyway if I'm spending the night somewhere.