Depends entirely on the situation.
1. Are they a danger to others? If no, move to #2. If yes, fire them and don't look back. If anyone gives you grief over it, fire them too just to set the record straight who the boss is, and isn't. This may hurt some reputation you may have, but over time the truth will come to light and things will work themselves out. Usually. Bonus: Future employees will know you don't fuck around.
2. Do ALL the other employees have an issue with this employee? If yes, have a group meeting to determine what the issue is without that employee present. Record everything, and let everyone know that they are being recorded and to be 110% honest about everything. If no, then move on to #3.
3. Is their work amicable or able to be improved at least? If yes, keep them, because coworkers can be backstabbing traitors who deserve worse sometimes. If this is suspected to be the case, move on then refer to other employees track records. If worse than others, fire them. If not the worst; work on improving them. Make it a group thing too so that no one feels singled out, because where there is one bad employee, there are others. Perhaps lesser in some way, but possibly worse as well. If their work is not amicable or capable of being improved upon, move on to #4.
4. Do they have any redeeming quality that helps the startup in a way others cannot manage to do? If yes, keep them regardless of #2 and #3, and find a way to make the scenario work for everyone. If no, continue to #5.
5. Alright, we've reached the final point. At this point it can be determined that the employee has no redeemability, and keeping them on has zero value for the company. Now we come to the question about length of time as implied by your question. The answer is, it doesn't matter. If the employee truly is a problem, cannot be redeemed or improved upon, and is even possibly a danger to others; just get rid of them. If however they can be redeemed somehow, or things can be made to work in the case of some coworkers not getting along and backstabbing each other; then keep them on and work through it.
Your employee could be a complete Wally from Dilbert, but still have some redeeming quality to keep them on the payroll and any benefits included. But if even the other Wally's of the company dislike this person, then it's probably best to just get rid of them and let those other Wally's know they are next if they don't smarten up.
Why do I pose it like this?
Because at the end of the day, you don't want to look like a pushover to the other employees that may be just as useful or useless as this employee. But you don't want to seem like a tyrannical asshole either. So a proper order of things has to be explored. It needs to be determined whether or not there is any potential way to make things work. If things cannot be made to work out somehow, then cut your losses and keep records if possible.
And as far as that vesting goes. Regardless of which ever decision is made to fire them or keep them; pay them out anyways. But make it abundantly clear why they are being fired, or kept.
My experience in the workforce has taught me that other employees will go to ridiculous lengths to get rid of someone they don't like, even if all those other employees are actually the problem. Thing is, groups tend to like to agree with each other, even when wrong. So it needs to be determined who is actually the problem, and who is just a problem to those other problems.
I was the problem for other problems. At one place, I was let go gracefully in a similar manner to how I just explained. And because of it, despite how much I dislike the bosses who fell for those employees bullshit, I don't run them under the bus and sue the shit out of them, because they are just ignorant fools who fell for malicious bullshit.
But there is another job I may be running under some monster busses here soon as soon as I can afford to get a lawyer who is willing to take my case. I've had to deal with some really shitty coworkers who should never be allowed to hold a job in that field ever again. But because of how people can be very convincing to fools; I was seen as the problem.
I hope this is a good enough answer for you, because I learned a long time ago that sometimes high maintenance employees are well worth their maintenance... sometimes.