I am sorry that you have been waiting for a response. I recently just logged onto the site and noted that your question hasn't yet been answered. I am a licensed veterinarian with over 25 years’ experience and would be happy work with you if you are still needing assistance.
I would not give more than twice per day without knowing if she still has stool in the colon to be passed. If she does, it may be that a glycerol alone is not enough. Glycerol works as a local irritant, is not dissolved or absorbed to any degree so is fairly safe, but the problem with these cases is that often there is a large piece of rock hard stool blocking the passage of anything behind it. Sometimes a little liquid or a small piece of stool will make it's way around it, but until that obstruction can be manually broken down and removed, most drugs will not be too effective.
If she already passed a stool and a hairball today, I would try to increase her fluid intake today and she what she can pass on her own. You can try adding broth or tuna water to her food and make it as soupy as she will allow as well as making sure she has access to fresh water. Canned pumpkin is a good source of fiber and may help her to keep things moving as well. If her stools have been hard, the human product Miralax® powder is very effective and safe. Use 1/4 to 1/2 tsp mixed in with canned food twice a day. It is an agent that draws water into the stool to help keep it soft and easier to pass.
If she is still not able to pass stool on her own, before giving any more glycerol, I would have her examined to see if she may be obstipated in which case she may need a manual evacuation.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.