For some stones that are more porous than others, one application of sealer may not be enough.
But how will you know? On mercantile granite that needs sealing, at least two applications are recommended. Very porous mercantile granite, sandstone, quartzite, etc. may require three or more applications. When sealer can no longer be absorbed by the stone, the stone is adequately sealed.
How long will it last? There is no absolute rule of thumb when it comes to the durability of any sealer. Generally speaking, most quality impregnating sealers interior will last 2-5 years or more. Environment plays a big role. Stones exposed to intense heat or direct sunlight will probably need to be re-sealed more often.
When is it time to reseal? To find out if your stone is perfectly sealed, spill some water on it and wait for approximately half an hour, then wipe it dry. If the surface of the stone did not darken it means that the stone is still perfectly sealed. Be sure to test various areas, especially those areas that get more use and abuse.