My recollection jives with Archie's. A month or less, and fairly variable for the genus. I think the taxonomy is a bit of a mess which may add to the reported variability.

Maybe the guy who posted quite a bit about his (gorgeous ones) will chime in - I think he was breeding them.

Here's a link to an abstract about the Okinawan one, by far the most well-known (and which some say may actually better "fit" in Protobothrops...). It has a blurb about incubation times:
http://sciencelinks.jp/j-east/article/200019/000020001900A0603263.php

Cheers,
Jimi