LinX/IBT only performs the error checking for stability purposes at the end of every cycle.
If you don't iterate through X-cycles then you have not stress tested your system to see if it is capable of repeatedly calculating the same set of computations X-number of times.
Precisely because the time involved per cycle is memory dependent (depends on how much ram you have installed and are testing), there is every reason to carry out the stress testing on a cycle-basis versus a time-basis.
The very same reasoning why you carry out ram stability testing on a cycle-basis (times through the loop test) which is also called "% coverage".
I can see the logic in that. But here's my argument:
If your RAM has already been proven stable with HCI memtest for example, then any errors made would be completely CPU dependent. So, if there were any errors made during the calculation, it would accumulate over time and reported at the end, not during a specific run. So arguably, you should run HCI memtest first for X cycles, so as to eliminate RAM from any overclocking issues.