HIMYM was the 1st show I started watching online from the US 7 years ago so I keep on watching even though it's gotten much worse ovee the years.It has already been mentioned that season 8 will be the last one. Now according to my knowledge they've had the who is the mother scene done ever since mid season 2. The one where he says it to the kids so not only has it been decided for years but we certainly will know.
Source: interview I cba to look up on my iPhone.