How much farm is enough?

Doesnt really matter tho if she/her illusions melt away. She really is a "all up in the enemies face" type of carry and playstyles like that rely on having some items ...thats also why heroes like Bristle Back and Timbersaw are considered "high risk - high reward".

We're talking about Spectre's early contribution, right?

Early, Spectre is like the clean-up crew. She can't really do much (unless you build her so [MoM, Diffusal etc.]), but as I said, Spectral Dagger allows her to chase very well. Desolate is also good at cleaning up when the enemies are scattering.

There is no arbitrary number of cs that qualifies as 'enough'.

You should continuously be looking to farm space across the map as a position 1, but you need to find a balance between joining your team for fights or knowing when your contribution isn't going to be required/or the fight is already over.

This is why i love spectre. You can farm and ulti in at an oppurtune moment to turn the fight around and get some kills.
For all carries, all heroes in general too, always carry a tp scroll to get to some fights. Later on a boots of travel pickup might also prove useful. Also, you shouldnt really just farm creeps all the time, if you feel an enemy carry is getting fat or see an enemy support all alone, move in with your team early on or by yourself later on to get kills and gold, killing heroes always will be better than just farming in the long run. This is also why i dont tend to rush items such as a radiance for spectre, maybe get a drums into a diffusial to have some early game potential and not come out too late. I once had an alchemist who spent about 40 mins farming, started helping in teamfights and just got burst down by an enemy phantom assasin. One such example of getting kills early was when i went a pa and ganked with my cm once i had my ulti. I was able to snowball and keep my farm potential throughout the game and keep them underfarmed.