If we are talking about a single shade in the palette, not the whole palette, then my answer is 1. If I did not like it, I will be wasting time on a nasty shade, when I could use something I like instead. Makeup expires, eyeshadows may change over time, better to use what I love. But usually I forget that I did not like something, and a year or so later I may try these shades again.
If we are talking about a single eyeshadow - several times. I sometimes buy singles to try an eyeshadow formula of a different brand, and I want to form my opinion on it, to understand whether I should buy from them again or not.
Regarding a palette - it depends. But I would try to use it at least a few times before deciding whether I like it.
I go for brown, beige, white, pinkish, orangey and greyish colours and mostly avoid everything else. Sometimes I use shimmers, but only occasionally. I use red and other bright colours in Christmas and when I prepare for celebrations, but avoid bright colours for my everyday looks. I also avoid very dark colours. If you give me black, I won’t touch it.