The simple answers are that at a minimum someone needs to clean the litter box and check food and water, and ideally it should be someone who likes cats, whom the cat likes, and who is willing to take some time to play with/pet the cat. I hire a friend's teenage kid for this.
The long answer is that it very much depends on the individual cat. I've known cats who panic every time their human opens a suitcase; I've known cats who could be trusted alone for five days (beyond which I have to agree with them that the litterbox gets pretty vile).
I'd suggest a catsitter visit every other day as a minimum until you know your own cat's tolerances very well. More is better, especially if the cat might have any medical issues. Hiring a trusted teenager, especially one who likes cats, can be fairly cheap. (I pay $10/day, since I can afford it, it's worth that much to me, and it's partly an excuse to encourage the kid to think about earning and saving money -- but I'm sure I could just ask for it to ne done as a favor, or any price in between .) If you don't know anyone you trust with a key, hiring an adult cat-sitter or boarding the cat are good options though more expensive.
The alternative is to admit that your lifestyle is nor currently suited to being a pet owner. Or to figure out how the cat could comfortably travel with you; sometimes that can be made to work.
(Yes, I saw the twice a day recommendation. Honestly, unless you know the cat has health issues I consider that overkill. Not a bad idea, just hard to justify on a price /benefit level. YMMV.)