My DM was in a nursing home after an aneurysm, and had a mini-stroke. As a result she was unable to eat or drink and was put on tube feeding. I live abroad so I didn't really realise what was happening, but she became non-communicative and was more or less comatose when we came to visit.
But...she had had an active life and despite the aneurysm was fit as a fiddle, strong as an ox. So what tube feeding does is to give you exactly the right amount of nutrients to match your very low amount of energy burnt. And, unless you suffer from some other ailment, it will maintain the status quo in your body - for a very long time.
So my DM was like this for a number of years, with a non-existent quality of life. And the nursing home just said that this was the way it was and there was nothing they could do except continue. We didn't have an LPA either. Cynical me now understands that the nursing home was just milking the cash cow for fees...
After a few years I switched nursing homes because I wasn't happy with the attitude, then on one visit I happened to bump into her GP. He said, why don't you write to the home to express your wishes that if the tube comes out, it shouldn't be put back in. I was not happy, if the previous nursing home had told me that I could have written it about 3 years previously.
As I understand it, a letter of wishes doesn't have the power of an LPA, but it still has to be taken into account. What I didn't know then was that actually the tube comes out fairly regularly and it's just standard to shove it back in. I thought that maybe I'd hear something in 6 months time, but actually it was barely a month before the tube came out and I got a phone call.
I stuck to my guns and said we didn't want it put back, and flew back to the UK. The nursing home gave her water but slowly she drifted away. Once or twice she seemed to perk up a bit, I've seen that mentioned a few times about dying people. So I'd go in every day and just read out loud from one of her books. And one day, she just died. I knew it had happened, her breathing turned into a slight rattle and just stopped.
Anyway, the message is...I think that tube feeding is inhumane, it can go on for a very long time and it's difficult to stop regardless of the quality of life that is being sustained. So with 20/20 hindsight, I wouldn't have agreed to it starting, and/or would have requested not to replace the tube years before I actually did.
And to answer your question, sips of water, and about 10-14 days.