To answer your question, excel works, like, right away. I didn't use a syringe, but I dosed my whole tank double the recommended concentration. Then one more treatment two days later at like 1/4 that amount. Couple of days later all my staghorn algae (lots of it..long strands) was bright red and my hair algae (what little there was) was dead also. There was also a patch of bba and it turned red as well. Took about a week before the (now red) staghorn algae disappeared. I'm not sure what actually happened to it. It turned kind of clear and then I suspect it was nibbled away by my livebearers.
Anyway, tank is completely free of staghorn and hair algae now after just two doses (and no follow ups) with no adverse effects on plants, fish, or inverts. Been several weeks now and the algae hasn't tried to make a recovery yet. Excel is a really fabulous product. A few cap-fulls will do wonders on an infested tank.
Of course, getting the optimal nutrient/light mix is the best way to ensure it doesn't come back, but excel is a great algae killer, and I suspect much safer than H2O2.