First of all, never hear of HDmonitor. Hwmonitor, but not HDmonitor. Secondly, it's not used for stress testing, it's used for monitoring sensors including temperature and voltage. For stress testing you use Prime95, Intel Burntest or OCCT. Prime95 on small FFT for four to eight hours is fine for most people, 24 hours to be absolutely certain if you know you're going to be severely stressing an overclocked CPU in daily use.
Are you stress testing due to an overclock or what's your reason for it?