Hello.  I'd hoped someone (a specialist) would come along to address your question.  I imagine there is no one set time limit.  My father, who suffered in his last six weeks from multi-infarct dementia (now called vascular dementia), lived for about 9 days, give or take a day, with an IV.  He was dehydrating the whole time, and it was clear to me that he was dying, though his doctor denied it.    You are facing a very hard time.  This site has some really informative share posts and questions/answers.  You can find them on this topic of your question; I did a brief search and found several comments about alzheimers and IV use and feeding tube use.  Use the search engine at the top of the page and type in search terms you consider relevant.   Best wishes to you as you work your way through this difficult period.