I'd say, at the rate we're going, we'll probably find out what the next denominational letter will be before February rolls around.
I wonder if the new varnish format is designed to make the newer bills last longer? It would make sense, since $5 bills go through so many hands and are very heavily circulated, going through so much wear and tear.
The $10 bill may not rely on such varnish, since it has very low demand and goes through so few hands. It's no wonder many vending and change machines have no problem accepting tens, but not fives.
It seems as if the reason for the latest surge of fresh $5 bills implies that many of the oldest $5 bills (HB* prefixes, mostly) released during 2013 and maybe 2014 are due to be replaced, since they are likely worn now. This likely means the life expectancy of a polymer $5 bill, at first, has been roughly six years. The new varnish on the newer $5s may make them last nearly a decade, possibly.
For what it's worth, when the new $5s came out in 2013, at first I saw heavy exposure of HBJ prefixes - which, today, I seem to see far less often now. Maybe those older HBJs got replaced by the later HC* prefixes (possibly HCS) which were printed a few years ago.