I'm putting together some charts and the time axis in one shows billions of years, the time axis in another shows millions of years. I've seen in various publications $my$ and $myr$ (also $ma$) for millions of years (ago). I've also seen $Gy$, $Ga$, $Gyr$ and $bya$ for billions of years.
I tried digging up an SI unit reference but didn't have any luck. Is there any accepted version of abbreviating large units of years that is consistent?
It depends on the field. There is an ISO standard to use a from latin annus, but in astronomy and cosmology yr is far more normal. I think the same is true in geology and paleontology. The metric symbols should be capitals, M not m (which stands for milli-, $1/1000$). b is deprecated for billions, use G. ya is often used for years ago. I use Myr, Mya, Gyr, and Gya, which are well accepted and unambiguous. They are also immediately intelligible to people who don't read ISO standards and don't speak latin.
It's Ma for million years and Ga for billion years.
