Once bowel incontinence starts, what is the average life expectancy for someone with Alzheimer’s?

There is nothing average about dementia.
Every case is as individual as the person's own fingerprint.
Bowel incontinence and urine incontinence has more to do with the portion of the brain affected, the health of the sphincter of the individual, the diet, even whether the individual has back pain (believe it or not) than it does with the dementia itself.

Again, your MD is the best person to give prognosis. Not a Forum. The MD knows the individual and any person comes with his or her own medical history.
Wishing you the best.

If you’re wondering about caregiving someone yourself after they become bowel incontinent - IMO, that’s when they need to be in a facility where they can get professional care.

Go ahead and search for a study. I will be surprised if you will find info. Based on my experience with 3 family members, you will be able to know when death is near when there is measurable weight loss, poor eating and poor drinking. Remember that many with dementia die from their other health issues.. otherwise, life expectancy with dementia can last 10 to 18 years.

On a personal experience, 2 of them went on for 5 plus years with incontinence

My Aunt had dementia for 12+ years and died from complications from a broken hip at almost 101. She was hardly bowel incontinent when she passed, ususally was able to tell you she needed to be walked to the toilet to have a BM.

I don't think there's any correlation between when bowel incontinence starts and when someone passes from dementia or other age-related issues.

I'm not sure that there's a correlation between bowel incontinence and end of life with someone with any of the dementias, as it is just one more thing that your mothers broken brain is forgetting to do and can no longer handle.
Might be time to get her placed in memory care if she's not already there.

I found incontinence can happen for many reasons, (diet, medications, dehydration). My mom had issues long before she lost her memory.

My mother in law had dementia and was on Aricept ... and it was determined that the Aricept was causing her to have diarrhea - and incidents with incontinence. The time of day at which the medication is taken makes a difference also.

GiGigrl: Therein lies nothing "average" about dementia; each individual is different.

More reliable signs that someone's body is shutting down are excessive sleeping, not eating and not drinking. However, even they could be caused by other health issues.

Bowel incontinence could be a sign that the body is failing, or it could be that there is pressure on the sciatic nerve, or a different medical problem.

Speak to the doctor and look into what other help you can access.
It could be time that you have your mother placed in care and others take over the day to day duties.
Don't risk your own health.

Incontinence is not an indicator of impending death. Something else may be going on. Talk to your loved one's doctor about this problem so the doctor can evaluate and treat.