I'm just wondering if when creating a song what's the correct way to time things. For example, I say the "chorus lasts 30 seconds" or the "chorus lasts 8 bars"
Either could be correct - or more helpful - in different situations.
If you are mixing down an existing recording and programming in some mix automation, it may be more helpful to think in terms of seconds.
If you are giving some musicians instructions before a performance of a conventional piece with a traditional time signature, it's highly likely that thinking in terms of bars will be more helpful both because it's easier for the performers to count, and also because the duration in seconds may not be known at that point.
If you have composed an avant-garde ambient piece with no obvious beat and no conductor or other band-member providing a count of bars, it might be reasonable to make each performer watch a stopwatch and work in seconds - it would be unusual, of course.
Would I say this beat that I made a loop on "4 bars" or a loop of "16 beats".
Again, it totally depends. If you are working with reference to a machine or a piece of software that only counts beats, 'beats' might be better. If it counts both, 'bars' might be fine. If you are lining the beat up with an existing piece that's notated in bars and beats, 'bars' might be slightly better as it gives a sense of lining your beat up with the bar boundaries.
I'm just wondering what's the convention
None of the terms you've mentioned are unconventional, but each of them is more helpful and relevant in certain contexts.
As it happens, in the musical activity I'm doing at the moment, it makes sense most of the time to think in terms of sample numbers @a sampling rate of 44100Hz. That would be crazy for most purposes, but it's the right thing for this particular context.