I will always support the author in the middle (about 15-20 episodes in, on most stories I read) and at the end of the story. I have just noticed a story I am reading is asking for gems to read the next episode early or I have to wait 13 days! And there are 2 episodes to read.
If I’m honest this puts me off a bit. I read a lot of stories and try to spread my gems to as many community authors as possible. If I have to use them all to just read the next episode, other authors will miss out. I did actually stop reading a story that did this but it was early on and I wasn’t invested, unfortunately I quite like this other story 
Edit: If the support author option is over 10, I may only support once.