My guess is that the grape acts as antenna for the microwaves. Because of the shape of that grape, a very high field is created at the tips. This field is high enough, that the air is ionised, leading to a plasma. Without the glass, the plasma quickly thermalizes witht the surrounding air. With the glass the plasma exists longer.
How much plasma is produced should be determined by the time the grape is able to act as antenna. The plasma will damage the grape until the effect doesn't work anymore. There isn#t much one can do about that.
For a longer effect, I'd try to use a thicker glass that is able to contain the plasma longer.
This is all pure speculation, though.