One of the things that comes into consideration when writing a play is "How long should it be?". On the face of it that sounds an eminently daft question to which the answer should be "As long as it needs to be." But there is a reason for the question which involves an audiences ability to concentrate and the playwright's ability to juggle all the elements needed to tell a story well.
When I started in the theatre back in the sixties, an evening of drama would be expected to last around two and a half hours with at least two intervals for drinks at the bar. Bar takings were an important way of subsidising what went on on stage. I've known customers ask for their money back if a play ran under two hours. And let's face it, Shakespearian audiences expected four hours with a Jig ( a shorter comedy thrown in for good measure). Of course, all of that was before TV came on the scene so audiences today are desperately consulting their watches to see if they can be home in time for the "I'm a Celebrity" catch up. Two hours is now a maximum and I have seen pieces down to an hour and a half and even an hour. I suppose Opera audiences are still able to concentrate for longer and there are some honourable exceptions in the theatre but these are actually flagged up with warnings about their length. I saw a magnificent production of "The Wandering Jew" some years ago at the National which clocked in at four hours.
But for me, an ideal length is around two hours for the sort of story I want to tell. There is usually a bit of a pattern I like to follow with a main turning point half way through and several smaller twists on the way there and back. So that readily falls into two acts with two or three scenes in each act with the climax coming just at the interval. The number of scenes varies but I don't like to have more than three per act or the steps become too complex and we lose audience focus. Anything over a total of eight scenes becomes a television drama in which audiences have to rush in and out to the kitchen whilst watching and so can't be asked to watch for more than two minutes at a time. The beauty of live theatre is that the drama unfolds before your eyes at an appropriate pace not driven by the next ad break. Longer scenes give the characters space to breathe and demonstrate complexity and development.
So given, the format I like to work in, and given that I aim for around two hours running time I find that my best guide to length is word count. And I reckon that at somewhere between 15,000 to 20,000 words including stage directions. (Usually stage directions last as long on the stage as they do to write). Of course, this is a bare guide but it does give me an indicatiion that I've given the characters enough room to develop fully without become windy and verbose.