Yes, it is 8 seconds for everyone because, give or take, that’s how long a human can hold an isometric contraction before the muscle is depleted of oxygen and then the effort is anaerobic.
“What’s wrong with that” you might ask. The answer is, technically, nothing… But…
As previously mentioned the QL is a massive spinal stabiliser. Aside from moving your spine it also helps prevent buckling and helps avoid too much stress going through our discs. If these muscles fail due to fatigue the result can be poor form, loss of power (on the bike) or, the results could be much worse (think herniated disc).
Now, in almost ALL activities (be that sport, work etc) they almost exclusively fail due to endurance fatigue and NOT because of a lack of outright power (with the caveat here being power lifters etc, but that’s a whole different kettle of fish). Coupled with the fact that in almost ALL times these muscles fail or become fatigued in an activity it is extremely rare for that to be because of a static hold (isometric contraction). When we cycle, the muscle is switching on/off/on repeatedly… So therefore it makes sense to train it that way.
Why does this matter? Well, because if we train it anaerobically it will not develop particularly well at endurance, endurance being the most common form of failure. Therefore, doing reps of 8 seconds avoids anaerobic work and is the best way to build endurance.
Going to the “I can’t reset my position in 2 seconds”… You can. I’m not necessarily talking about going from a hold to a complete rest and back to a hold in 2 seconds, a fairly significant movement to get you to that “almost at resting pose” is enough to alter the muscle contraction to add venous return.
Because of this, 8 seconds on, short movement/reset and then repeat is better than doing, say, 1/2 the reps. But 15 second holds.
I appreciate that’s a long reply but I hope that answers your question.
Ultimately it is up to you what you decide to do, but this is what I tell all of my patients, even the fairly high end sports people I treat. This is also what Stuart McGill says… And he’s about the most knowledgeable person when it comes to lumbar core stability in the world.