It depends, seriously, there really isn't a solid answer to this question because the language you are work with matters, the five to fifteenth lines mentioned in this answerin this answer might work for C# or Java, but in other languages it doesn't give you much to work with. Likewise, depending upon the domain you are working in, you might find yourself writing code setting values in a large data structure. With some data structures you might have tens of elements that you need to set, should you break things out in to separate functions just because your function is running long?
As others have noted, the best rule of thumb is that a function should be a single logical entity that handles a single task. If you try to enforce draconian rules that say that functions can't be longer than n lines and you make that value too small your code will grow harder to read as developers try and use fancy tricks to get around the rule. Likewise, if you set it too high it will be a non-issue and can lead to bad code though laziness. Your best bet is to just conduct code reviews to ensure that functions are handling a single task and leave it at that.
