That is quite open ended and really depends on the specifics of your project. For ex in a game -
• How close the asset will be to the camera? Something the player interacts with constantly or sees up close usually needs more detail to feel good.
• What is its relative importance compared to the rest of your scene?
• Are you trying to reproduce the look of a distinct retro system?
• Are there any other technical constraints you need to consider?
As an example, on ULTRAKILL regular enemies tend to be 1-2k tris and their textures rarely go above 256x256, but it really depends on whatever suits the needs of the character.
It would be a good idea to make a couple test / prototype assets to hone in your artstyle and requirements. The good thing about Substance Painter is you can always edit the model/uvs after the fact, reimport to Painter, and your painting will reproject onto the updated asset without needing much rework.
We often take advantage of this on UK and go back and forth between modeling and texturing to get things feeling right.
Hope this helps!