How long can I leave my cat alone?
While there is no rule set in stone, a rule of thumb is that 8 hours, e.g., while you are at work is ok. Importantly, it matters how and where you leave your cat alone during the day. The more space the better; make sure it is warm; that there is a place to rest; make sure there is clean water and food. Leave something for the cat to play with or explore while you are gone (e.g., leaving the TV on is better than nothing).
If you intend to leave your cat for more than 8 hours, consider that cats are individuals, just like human beings are. Some cats are more ok with being alone than others. But do remember that cats do get lonely. It also matters how often you intend to leave your cat alone. In addition, the fact that something is possible, does not mean that it should be done. How would you feel after 3 days alone in an apartment with nothing to do? In general, ask a friend or neighbor to check on the cat. Another option is boarding your cat in a cattery. If the cat is left alone for too long, it can leave to behavioral issues like aggressiveness or clinginess; but even if it does not result in observable changes, it is not nice to the cat.