How often are sales stats updated on iTunes Connect?

Around 3am ~ 5am PST daily when the system is working normally has been my experience.

According to Page 17 of Apple's latest documentation,

Reports are available by 6 a.m. in each territory’s respective time zone.

In the US, this would be PST. In Europe, Africa, and Asia, this is in CET. In Japan, it is JST. And in Australia and New Zealand this would be in AEST.