Describing something as 'getting old fast' is a very common phrase in English to describe something that as tedious, but it has the nuance that the thing may not be boring initially. Is there a natural equivalent in Chinese?
'beating a dead horse' will make the topic 'getting old fast'
The core meaning of 'getting old fast' is 'action or words become repetitive and starting to bore people'
The equivalent in Chinese would be 千篇一律. It literally means "a thousand song with the same tunes" (it would certainly getting old fast and bore people). That captures the essence of the English expression "getting old fast" pretty well. However, the common usage of 千篇一律 nowadays is to describe 'uncreative'
A close equivalent in Cantonese is
「講嚟講去三幅被」(talk and talk and it is all the same), but it only refers to 'repetitive boring talk' not including 'repetitive action'
形容人講嘢重重複複，毫無新意 - describes people repeating words repeatedly, without new ideas.
The problem with matching English and Chinese idiom is that no matter how close their meanings are to each other, there would always be some stable difference gets lost in translation.
This blog post recommends two general translations:
The author also gives a suggestion of:
不耐 + V.
For an online game getting old real fast they translate it:
You need to be selective with this one because 不耐 doesn't work with all verbs.
Another suggestion is to use a phrase like:
Sometimes the answer in Chinese is too simple that you wouldn't even have thought of it.
Colloquially, we say 两天半新鲜. E. g. I bought a new iPhone. I like it very much at first. Then after not very long time, I don't like it as before. In this case, we say 也就两天半新鲜.
It's an expression that is used to describe that the liking only lasts for a very short time(两天半).
It's not an idiom though, but a common colloquial expression.