That really depends.
If everyone voted right now and there was a majority, then the day would be over, and it would become night immediately, and everyone would preform their night actions. And if everyone did those immediately, and repeating, then it could be over in a week or less.
However, if no majority is reached, then the game will default to most votes at the end of the "day", and it takes much longer, and same with night (though night is almost always shorter), and if that continues it could be weeks or even, in some cases, months, before the round is over.