That’s a good question and it depends on your goal. I know you mentioned powerlifting, but in the general interest, I will cover the topic more broadly.
HYPERTROPHY: The instinct would be to use a duration of 20-30 seconds for hypertrophy stimulation. The problem is that if you do one long effort lasting 20-30 seconds you will likely not accomplish much or cause problems.
First, it is impossible to maintain a maximum contraction for 20-30 seconds. In fact, even 6 seconds is pushing it. The first thing that people might do is push/pull as hard as they can for the whole set. As fatigue sets in, force production gets lower. Which is not a problem in itself when growth is the goal. The problem is more about giving all you’ve got for 20-30 seconds. This is an amazingly high stress on the nervous system, especially as one continuous effort. Imagine grinding a max effort squat for 30 seconds!
The second thing people might do would be to go easier at first to make sure that they can maintain a fairly constant force production for the whole set. Which might be better, BUT it’s very hard to know if your force production is high enough (it would have to be 80% of your potential).
The protocols I like for hypertrophy are:
6 x 6 seconds. Your set has 6 “reps” lasting 6 seconds, with 10 seconds between “reps”. The reps are done as follow: The first 3 seconds you gradually ramp-up force production and the last 3 seconds are an all-out effort. This is normally my go-to method for hypertrophy.
15-10-5: Here you have a set lasting 30 seconds but there is an effort gradation. The first 15 seconds are done at around 70%, see this more as an activation or “contraction practice/pre-fatigue”… you then rest 10 seconds and do a 10 seconds effort at around 80-85%. This should be hard but not like you are getting out of your shoes… rest another 10 seconds and the last 5 seconds is all-out. This is a very good approach for people new to isometrics, and it also better respects the recruitment pattern found in a regular set.
POWERLIFITNG/STRENGTH 1: If you want to use the Hoffman protocol (2-3 positions per lift) to strengthen the whole range of motion I recommend measuring the duration of the concentric portion of a max effort on your lifts and use that as your duration for the isometrics. For example if on your bench, you need 3 seconds to lift the weight, then your max effort on the isometric should be 3 seconds.
If you want to be even more precise, also measure the duration of your eccentric and make that your ramping-up duration. For example if during a max effort squat the eccentric lasts 2 seconds and the concentric 3 seconds, when doing your isos each set should last 5 seconds: the first 2 you ramp up force and the last 3 are all out.
This is to respect the neuro-dynamics of a max effort rep. If you want to play it safe you might even make the iso 1 second longer than your max effort. The key is teaching your body to maintain a maximum contraction for the required period to complete a max effort lift. This is especially important for those with sticking points later in the range of motion. YES sticking points can be a matter of lagging muscles. But it can also happen when the rep exceeds the duration for which the nervous system can maintain a maximal contraction.
Now you might ask: “yeah but if I’m using 3 positions, shouldn’t I divide the concentric time by 3 so that each position will be under tension for the duration you would spend in that position during a lift?”. No, it’s about training the central nervous system to maintain a maximum output for a certain time.
If you don’t want to measure your concentric and eccentric time. The 2-3 seconds ramp and 3-4 seconds max effort is normally where I would start.
POWERLIFITNG/STRENGTH 2: You can also use the isos to strengthen a weak point. In that case you only use one position, just above or just below the sticking point. In that case the ideal position and duration will depend on the type of lifter that you are.
If you are a lifter that uses speed/acceleration to overcome a sticking point (more explosive than strong) do the iso slightly below the sticking point and use a very short effort, 2 seconds BUT don’t ramp into it SLAM the bar from the start and push as hard as you can for 2 seconds.
If you are a good grinder (stronger than explosive) do the iso slightly above the sticking point and push for 6-9 seconds with a 3 seconds ramp-up in force. Start at 6 and gradually work your way up to 9. Do not increase duration if you feel your strength/force decreasing at the end of the iso.
POWERLIFITNG/STRENGTH 3: The final use of overcoming isos for strength is as an activation tool. You can use it prior to a max effort or a heavy lift. In that case select the position at might you can produce the maximum tension (likely 100 degrees joint angle, elbow or knee/hips depending on the lift) and go all out for 1 second with a 3 seconds ramp-up.