When did the writers plan Monica and Chandler were going to get together in Friends? I am wondering how far in advance it was planned, as there's such a long history between the two characters.
How Friends Decided to Pair Off Monica and Chandler gives details. It's a big article, but the part may interests you that:
Friends was nearly 100 episodes into its run before the show's writers played the Monica-and-Chandler card, but the coupling was something that almost happened earlier. The idea had been "kicking around" since before Silveri and Goldberg-Meehan joined the show in season three, he remembers, with his predecessors taking note of the chemistry between the two characters as early as the season-two episode "The One Where Ross Finds Out." That plot had Monica acting as a trainer for Chandler as he tried to shed a few excess pounds, and "there was a real fun dynamic between the two of them," Silveri says. "So even as early as that, they said, 'Oh, they're kind of special together. If we're ever looking for another relationship, that's something to file away.'"
The notion of Monica and Chandler was also seriously pitched in the writers' room in season three, Silveri says. "People got excited about the idea," he says, including himself among that group. Goldberg-Meehan, however, thought it was simply too soon in the show's life to introduce another couple. "She was the one who said, 'I just feel like at this point it would feel a little desperate,'" Silveri recalls. "We had gotten excited about the stories we could tell, but once she said that, we were all shamed and ran away. It became clear it was too early to explore something like that." Another reason the writers put a pin in the idea: "There was a little bit of relationship ennui among us writers," Silveri says. "We'd already done a lot of drama between Ross and Rachel. And nobody wanted it to become the 'Get Together and Break-up' show."
It must be during or before the season when they got together. As with any show, no one can plan or predict in the beginning how many episodes or seasons it will get.
One thing for sure, I heard this during the documentary of 9th season of Friends: When the 9th season started, they had plans for it to be the last season. But during filming of this season, they decided to have another one. That was the reason (or one of the reasons) 10th has only 18 episodes whereas all previous ones had 24-25.