I know this is an old thread, but I got here trying to answer the same question, and there is a WIDE range of dilutions given depending on who you ask, where you read. Just a quick summary, in case some of you end up here searching like me (bear in mind I'm a noob, I can't vouch for these numbers, they're just weighted based on which I found more people confidently suggesting elsewhere...)
1) Most ppl say don't use H2O2 with organic nutes, although this is debated. IF you use organic nutes, definitely do your research and make up your own mind before just tossing Hydrogen Peroxide in your rez.
2) The number I found most commonly, at least for a DWC hydro system and 3% (store-bought) hydrogen peroxide, was 1mL H2O2 / 1 gallon of water. Slightly less is acceptable as well (10-15 drops).
3) I found other growers who use as much as 15mL per gallon -- seems awfully extreme by other standards, but might give you an indication how far you can go before causing any serious problems (emphasis on MIGHT).