Generally a sleep will last at least until the next system interrupt occurs. However, thisdepends on settings of the multimedia timer resources. It may be set to something close to 1 ms, some hardware even allows to run at interrupt periods of 0.9765625 (ActualResolution provided by NtQueryTimerResolution will show 0.9766 but that's actually wrong. They just can't put the correct number into the ActualResolution format. It's 0.9765625ms at 1024 interrupts per second).