NPO

If they need to be NPO for more than a couple of days then they need to be on TPN

so patients can survive only on IV fluids for just about two days?

Starting alternative forms of nutrition is recommended for many patients starting on day 2 because the lack of nutrition can begin to have clinically significant effects after two days, not that they'll will likely day on day 3 of being NPO. How long someone can survive a fast depends on their nutritional levels at the beginning of the fast, but being able to only survive two days would be pretty rare.

There are a lot of factors to consider each patient is different. I have seen patients go over 5 days before TPN was started. But as the previous poster stated, the nutritional status of the patient before becoming NPO.

If the patients condition is looking like they have to be NPO for more than a couple of days there should be discussions around when/if a patient should be started on TPN or PTPN

I see recommendations to start TPN or TF usually at 3-5 days.

yes they can.