"get well soon" OR "feel better"

When to use “Get Well Soon” and “Feel Better”, I mean in what situations? Should “Get Well Soon” be used only when person is unwell for many days? And should I use “Feel Better”, if colleague message me that she will take off as she is not feeling well.

Both are fine in both circumstances.

Neither phrase requires that you are certain of the diagnosis - "I'm not feeling well" is sufficient indicator to allow you to use both terms. In both cases you are simply expressing a hope that the illness or "feeling unwell" passes quickly (with an implication that whatever is causing it is remedied soon)

The only time "Get well soon" would be a bad choice is if the person you are speaking to has a terminal or incurable disease, in which case hoping they "Get well soon" makes little sense (they will not get well at all). In this context "Feel better" is quite casual though too, and has a similar issue... you would likely be better expressing your sympathy in another way in such a scenario

I'd also note that while they're used almost interchangeably in the US, "Feel better" is very much an American phrase. In the UK it would be far, far more likely to use "Get well soon" or "I hope you feel better soon, while "feel better" would likely be seen as flippant or insincere. I'm not sure about Commonwealth or other English speaking countries.

I have not heard 'feel better' used, so I assume it is modern, casual and colloquial and therefore is most appropriate in less formal situations. 'Get well soon' seems to be used fairly universally and I can see no reason why it would not be applied in a situation where someone has suddenly become ill and had to leave. Of course, as remarked by a previous person commenting, one would not say it to someone with a terminal illness, or a chronic illness that had no likelihood of a cure. That would just show your ignorance of the situation. I am in Australia, so that could explain why I have not heard the expression 'feel better', but it could be because it is being used by a younger generation than mine.