I have heard that you should spend 1 hour for every 1 minute of script. This might be a little excessive, depending on the play. However, its not a bad barometer to base your time estimate off of. Its important that you spend enough time going over the play, this is from book work, to blocking, to exploration. Its possible (and I’ve been involved in productions where this happened) to under rehearse. Whether this means that you just didn’t allow yourself enough time, or that you didn’t explore enough. I think an hour for every minute is a good starting point. This of course, increases if you have stage combat, or music, or dancing.