Depends on where you are.
Some of the old POS ASP batons are still being used, so they'd be short.... 16" I believe.
I don't think anyone makes a swivel holster with rotational retention for ASP.
Some places have moved to Monadnock AutoLock batons with the power tip, some with the rounded end cap, some with the regular. They are a far superior product to the ASP steel batons or any friction lock product. It honestly depends on what was procured by your local detachments. For the most part, I've seen 18 and 21 inch batons, but there's everything from 16 to 26 inch. It all depends on where you are and what you're issued.