well, first of all, it doesn't decay, some of it disappear and the rest becomes lead in the end. But we're talking several million years, of I recall correctly

Depends on it's original potency yo...

For how many half-lives should radioactive material be stored safely before it decays to an acceptable level of radioactivity?

Different radioactive materials decay at varyinig rates, so it's impossible to generalize to just any radioactive material.

Depends on a variety of things. For example, Uranium-238 is radioactive, but with a halflife of nearly 4.5 billion years, it can ba considered relatively stable when compared with Cobalt-60, for example, with a 5.27 year halflife. Also, the type of radioactive decay is important, as well. Using the examples above, U-238 primarily decays by Alpha particle emission. Alpha particles are not very penetrating and are, quite literally, stopped by the dead layers of skin cells on your body. Clothing blocks Alpha particles. Co-60, on the other hand, is an emitter of extremely penetrating high energy gamma radiation. What is "acceptable" essentially boils down to when the radioactive substance no longer leaves a detectable footprint with respect to natural background radiation. (Plus a few other concerns, like chemical toxicity, how chemically reactive the material is, etc.) For most low level radioactive materials, 5 halflives would be a reasonable rule of thumb. Only 1/32 of the origional radioactive material will be left, or 3.125%. For more energetic radioactive materials, however, some significant risk may still remain due to the penetrating energy of the type of radiation being emitted. For such materials, longer storage would be required, and ideally additional processing to separate the remaining radioactive material from the non-radioactive material. Using Co-60 as an example, with a halflife of 5.27 years, if we start off with 1 kilogram (about 35.3 ounces), in slightly more than 26 years there would only be 31.25 grams (a smidge over 1 ounce) left. Chemically separate the remaining Co-60 and the rest can be disposed of as non-radioactive.