Hi Big E,
Try about 10% first, depending what you're dipping for.
Time exposure will also depend on what you're dipping for.
I normally use H2O2 (3%) to clean the polyps from unwanted algae when I first collect them. Nothings is better than H2O2 for algae. If there is no visible algae I use Lugol's.
I used to leave for like 3 minutes in the past, but even less than that works.
Today, for the algae, I leave for about a minute or so. Normally that is enough.
The less you expose the polyps, the faster they'll open after treatment.
If you see too many bubbles around the polyps you can even leave for 30 secs.
The main point is that there is no need to expose the polyps for longer than necessary.
You need to ask yourself why to use H2O2. Each substance has it's affects on different pathogens/algae/bugs and affects the zoas as well in different degrees.
Consideration on the actual condition of the polyp(s) is crucial when deciding what type of dipping method will be used. I aways try to remind people about that. Such an important point!
Grandis.