Is it okay to it ferment for a long time? How long can I let it ferment?

Yes! In fact, leaving your wine in fermentation, even for 2 weeks, is completely fine. Your wine will just get very dry, and it will be right around 13% ABV.

If you've tasted your wine and it tastes weak, or way too sweet, you should let it ferment for longer. Try to raise the temperature, as well. However, after 14 days, usually fermentation will slow considerably, so it's not always super helpful to leave it longer than that.

If you forgot about your wine, and left it out for 3 weeks or more, don't worry! It's going to be dry and acidic, but you can fix this. You'll simply want to backsweeten your wine by making a simple syrup and adding it gradually to your wine. A little bit of sweetness goes a long way to balance out the acidity!