IF there's such a thing as typical, how long would it typically be before the checking started to show?   

Checking is a spirit finish or nitrocellulose finish thing. I think new mfg banjos are using more poly finishes now, and those should never crack.

I rarely use a modern finish, but believe this is what I've read over the years. It is my understanding that modern finishes (i.e. lacquer) have improved and are more flexible, and this comes from the research and development of the automotive finishes. I'm sure others will offer more insight.

Part of that checking was because Gibson used a thick lacquer that tended to check. Look at older Gibson, resonators and the guitars also have a lot of checking.
Kg

Is there anything to do about checking? I believe that's the condition of my lacquered 1973 non-Gibson resonator.

As an aside, in the world of electric and acoustic guitar snake oil, there’s a perception that the thinner vintage nitrocellulose finishes allow for better resonance/tone. It’s always positioned as a selling point, and while it’s probably an extremely minor difference (if any), I’ll admit to going that direction on my guitars. I like being able to feel some of the underlying wood grain and I’m okay with the checking — it’s a right of passage for a beloved instrument. ;-)

I have an orphaned Huber resonator from 5/13 and I don’t know its back story, but it is checking worse than my 90 year old Gibson. Who knows? Maybe someone did it intentionally? This picture doesn’t do justice to how bad it’s checking, but there it is. Don’t get me wrong, I really like the way it looks. But I wanted to share since the subject was checking on modern banjos.

It may be a small factor, but I think our homes having more consistent temperatures than years ago, helps as well. I have always thought wide or quick temp changes was the main cause of checking.

I have always believed checking is induced by rapid temperature changes.

These accumulate over long periods for most pickers.

I once helped sell a 1928 Granada that had been found stored in an attic in Connecticut for years or decades. It had been roasted and frozen in many cycles. The (thin) resonator finish looked like a fried marble.

I have my dad's 1946 Gibson Southern Jumbo guitar, which he bought new around 1947-48 but was too cheap to spring for a case. It rode in the back seat unprotected through dozens of Maine winter cycles. It has very thick finish. The top and sides have an ASTOUNDING amount of finish checking and in places the finish has even flaked away. Oddly the back has the least checking, by a lot. I bought him a nice hard case around 1975 or so where it has rested comfortably for the past 50 years. Checking has not advanced in that time.

As for "modern" finish, my dad's 1967 Martin D 35 never showed any checking of note until maybe 10-15 years ago when it started to show up in the top. The guitar has always been kept in a case, never exposed to sharp temperature DROPS. But Dad learned the hard way two times that you can't leave a guitar in its black hard case in the back of a Chevy Impala station wagon in the blazing sunshine at a bluegrass festival -- because the bridge will come unglued. After two warranty trips back to the factory he smartened up. I suppose that might have contributed to slow development of checking. That guitar's checking is long lines wandering across the top, not the "fried marble" type. Again, back and sides don't show much.

I'd like to add that banjos/guitars with "overspray" is almost guaranteed to check significantly over the years.