I tried 3% H2O2 once and that was enough for me, nothing happened. The bottle was straight out of the supermarket. Was it degraded ?, I'll never know.
My advice is just go for the "real stuff" and find some much higher % solution. Even if that degrades over time it will still be strong enough. I covered my board with about a 1/4" and just rocked it from side to side, no brushing or rubbing with a sponge. Etched under 2 mins. Fast etching is good etching and reduces undercutting too.
You'll probably find it's cheaper for the actual amount of H2O2 you get to buy it in concentrated form so you win both ways. Remember 3% is 97% water.
Keep it refrigerated and in a light proof container, or wrap the container in silver foil to keep the light out.
Keith.