Oh, interesting question.

Well, I've looked into this a bit myself, though it hasn't made its way onto the timelines yet. We know Scalphunter's published adventures took place during the Civil War in the 1860s, since that features largely in the stories, and Ke-Woh-Noh-Tay (sp?) even arm-wrestled Abe Lincoln at one point!

Bat Lash appeared in that Scalphunter series in Weird Western Tales, meaning that Bat Lash's "origin" took place prior to that crossover in the 1860s.

However, both Bat Lash and Scalphunter have had crossover stories with Jonah Hex in the mid-to-late 1870s, so they were both active throughout that time. (Jonah Hex's adventures are decidedly AFTER the Civil War, since the Civil War figures largely in his origin story.)

And it is likely that Bat Lash probably lived to a ripe old age, as some Post-Crisis stories have shown.

I tend to assume most DC westerns take place in the 1870s, in the time shortly after the Civil War. It seems to be a safe bet.

(As a side note, the appearance of Cinnamon in that mid-80s JLA story that crossed over with Jonah Hex, Bat Lash, Scalphunter, and Cinnamon is a bit problematic. The original Cinnamon stories very clearly take place in the 1890s, which is 20 years after the JLA story was supposed to take place. In order to have Cinnamon appear in the 1870s when she would've been a child at most leads to some interesting ideas, such as time travel in the Old West, or Cinnamon having an older female relative who was active 20 years earlier, etc. I've got some theories about that, but haven't really delved into it too deeply.)