Alcohol content will grow longer you keep it fermenting... ;).. if you keep it too long going and the sugars are used up , yes it can have decent amount strenght but it will taste bad too...
Any liquid that has yeast and sugar mixed and allowed to ferment will produce alcohol..how long you allow fermenting to happen (and type of yeast and amount of sugar) is the factor of how much alcohol...
,,now scientific types will give you list of other factors but above is the basic rule... ::)