When did the writers decide that Dallas season 9 was a dream?

Question

I'm watching through Dallas, and I'm to season 9 (by the DVD numbering; I understand it was previously called season 8). Patrick Duffy left at the end of season 8. Season 9 is all a dream Pam was having one night, leading up to the infamous shower-reveal. What I'm wondering is, at what point in the process was it decided that season 9 would be a dream? Was this a last-minute thing? Or did the writers know it was coming at some point earlier in the season?

Essentially, how much of season 9 is written to be "reality" and how much is written to be Pam's dream?

Answer 1

From this Dallas Dream Season FAQ:

• Why is it termed as the dream season?
  
  Season nine followed the aftermath of the death of Bobby Ewing. When Patrick Duffy decided to come back as Bobby in the spring of 1986 the producers decided to make the whole of Season Nine a DREAM. So Pam woke up and Bobby was alive, it had all been her nightmare.

  [...]

• Why did they bother to bring Bobby back anyway?
  
  Ratings were down this season, this was blamed by Larry Hagman on the producer Philip Capice, Larry Hagman also missed working with his old friend Patrick Duffy. Larry Hagman did not hide his contempt for Capice and finally said either Capice went or he did. Capice left and Hagman also demanded Duffy back. Larry was not happy with the stories that season.

  [...]

• Who's idea was it to have the dream?
  
  Patrick Duffy's wife is credited with the idea. Apparently she turned to Patrick and stated "The only way you can go back to that show is if it turns out to be a dream."