Us Far 121.377
Whiskeyflyer,
What you are look for is this in the US:
§ 121.377 Maintenance and preventive maintenance personnel duty time limitations.
Within the United States, each certificate holder (or person performing maintenance or preventive maintenance functions for it) shall relieve each person performing maintenance or preventive maintenance from duty for a period of at least 24 consecutive hours during any seven consecutive days, or the equivalent thereof within any one calendar month.
Note that this is only for FAR 121 (airline) operations. I did not see the same rule in 91, 135 or 145. Some state's labor law require time and a half for every hour worked past 12 in one shift, so that might be more of a stopping point than anything else. But the way this rule is written, you could work 26 straight days, get 4 off, and do it again month after month after month.