A cubic meter of air weighs about 1.2kg. A cubic meter of 15% O2 and 85% He would weigh about 20% of that or about .25 kg. so a cubic meter of the O2/He mixture would lift about 1kg.
Ignoring the weight of the bubble itself, a 70kg person would need a 70 m3 bubble, which would be about 5 meters in diameter. Since the bubble material would have to be pretty strong to support an adult standing on it, let's call it as needing a 6 meter diameter bubble to also lift the bubble's weight.
Note: While floating in a bubble will attract media attention, it would be most unkind to ask the floatee to do interviews...
According to hosstuffworks:
The average adult at rest inhales and exhales something like 7 or 8 liters (about one-fourth of a cubic foot) of air per minute. That totals something like 11,000 liters of air (388 cubic feet) in a day.
The air that is inhaled is about 20-percent oxygen, and the air that is exhaled is about 15-percent oxygen, so about 5-percent of the volume of air is consumed in each breath and converted to carbon dioxide. Therefore, a human being uses about 550 liters of pure oxygen (19 cubic feet) per day.
So a human needs about 1 cubic meter of pure oxygen/day or about 5 cubic meters of the O2/He mixture. This website says that people can comfortably breath air that is 25% depleted of O2 (equivalent to 10,000 feet which by experience is no particular problem for healthy people.)
Given that, an adult needs about 20 cubic meters of the O2/He mixture/day so as to not deplete it below 75% of its original oxygen. The 6 meter sphere has about 120 cubic meters of the gas mixture, so it will last about six days without any difficulty and would probably work for double that in a pinch.
Hydrogen works the same as helium within the accuracy of these calculations, but a no smoking policy is advised.