Voting takes up a relatively small amount of time. In my case, just filling in a mail in ballot, so I didn't even need to drive somewhere. If that's the extent of the required participation every few years, I would do so even if the tangible benefits were marginal, since the costs are so trivial.
Some anarchists historically have refused to participate in voting to deny legitimizing the state. This is an ideologically pure and legitimate stance, but it's difficult for me to see what that achieves practically other than the ideological purity.
If I try to look as objectively and dispassionately as possible at the practical outcomes of democrat and republican governments in the US, the democrat governments, while still corporate captured and moving further to the right, does pass some legislation that has, at times, reduced suffering for some of the most in need. It is still completely insufficient, but for many, I'm sure it has made life more bearable, and in many cases saved lives.
Climate legislation has a similar result, with Republicans blocking all bills that could help, where as democrats were able to get some passed that, while insufficient due to still having to appease capitalist interests, are definitely way better than nothing. Seeing as we have so little time to impact climate change, I will generally prioritize practical outcomes more than ideological purity, because ultimately if global warming gets bad enough, there will be very little humanity left to be ideologically anything at all.
There are many other parts of society that would also very quickly suffer under this republican candidate and underlings in particular, such as trans people, immigrants, and women.
That leaves the genocide, which both parties will continue to participate in, and which makes voting for either party ideologically disgusting. Again, I personally try to detach my own feelings on this and to consider the practical outcome, which is that regardless of my choice to vote or not, that suffering and inhumanity will most likely continue, and my lack of a vote does nothing to reduce it. With that in mind, I only consider the things my vote potentially could change, which so far are still worth the 5 minutes I personally have to commit.
Ultimately I know that my vote only delays a fascist state, but it also makes it more survivable for some along the way, and that's not to be dismissed, even if the same group making it more survivable for some is simultaneously enabling genocide.
There's a lot of variables, and it's deeply unjust that I'm forced into a position to have to weigh these variables between greedy power hungry cretins who enable so much suffering, but that's what I'm left with. You could think of it as picking which enemy you want to face.
But as for your question of where the line in the sand is for me to consider it not worth voting; I would consider it pointless if either party would result in near enough the same amount of suffering overall, and the only difference is the flavor (a random example, choosing between a Soviet Union style authoritarian state vs a mafia state like modern Russia)
Or,
The election is so thoroughly corrupt that my votes, if counted at all, will consistently be rendered useless by an absurd number of fake votes to where the whole thing is a charade (modern Russia).
But that's just my two cents :)