The decision to extend will come down to ticket sales, reviews, award prospects, and/or the cast's availability. I don't believe another production is programmed into that space just yet for the fall, but that's the earliest the next one would be opening.
In terms of On the Twentieth Century's extension, I imagine it was so short because of how taxing the part was for Chenoweth. She's mentioned that she struggled pretty badly with her Meniere's Disease towards the end of the run. Box-office wise, I'm sure they could have run through the end of the summer had all parties involved been up to it.