There are a lot of different typefaces and a single rule doesn’t apply to all of them. Experiment with letter-spacing and do what seems right to you. There are some simple guidelines that will lead you in the right direction, especially when working with body text:

Letter-Spacing Headlines
If you are using well designed fonts, you can be sure that they are calibrated well, and you won’t need to make any major adjustments to them. However, the problem with headlines is that at larger scales the space between letters looks unbalanced. It can be fixed by increasing or decreasing the letter-spacing value.

There are no strict rules for letter-spacing — there are a lot of typefaces and all of them require an individual approach — but if you look at how big companies like Google and Apple treat their typefaces, you can find a lot of valuable information there.