You can cast them anytime, on anyone, but best time is when the Moon or Venus are in good and operative positions (not void, and not in detriment, and in certain signs). And after you've built up your energies.
Though if they have a strong aura, or knows witchcraft, then the spell may not work due to the protection they have around them.
You don't need to ask permission to cast spells. That a self-defeating mindset. And it defeats the point of autonomy. But if you're asking for a moral guide, then good luck, because you won't get a consistent answer because everyone has their own opinions on it.
People will always try to talk circles around it, but the fact is that it works and we're given how to use it. Whether you want to do it is up to you and what your opinions on it are, and what your unique situation calls for. Is this person single? In a relationship? Do you care about their relationship? Are they mentally stable? Do they have a record of abuse/manipulation/cheating? Are they good at relationships? Are they reliable? Are they what I want in a person? Do I just want to have a hookup with this person? Would it be quicker to ask this person out instead? Is casting a spell really worth the trouble?
These are all rhetorical questions. You're free to care/not care about these things.
You have to be careful what you ask for, otherwise you might just get it
And when you make that spell, realistically who's there to stop you from casting it? No one.