Depends on the CPU you're running. Some will tolerate up to 75C, whereas some will have to be below 55C on constant load. Check the thermal limits on the manufacturers site (AMD or Intel)?
Personally, I wouldn't run any of my rigs past 60C on constant load. With relatively standard cooling, its not hard to stay below 60C. However this does not apply to laptops, as they have small fans and are built to tolerate up to 100C.