Glass delivery is the challenge here yet its not only how you get the glass in that matters but how to allow the air to get out. Its rather straight forward really, turn the piece upside down and look at where it may trap the air. Then place your air ways in the appropriate spots.
Time is then the issue, how long do you allow the piece to soak so that all the air finds its way to an air way? I have found in general that 10 - 12 hours will normally suffice, this is also called "the benefit of a doubt" as air can get stuck to the wall of a mold and decide that it likes it there in which case there isn't any remedy apart from vacuum.