1. The article says that Page was the "only American to have had his communications directly targeted with a FISA warrant in 2016 as part of the Russia probe." That's a significant data point in itself since this appears to show categorically that none of the other frequently mentioned players were surveilled directly.
2. The warrant was apparently obtained in July 2016. At one point the article refers to the FISA warrant being obtained "last summer". But later it says the application for a FISA warrant "showed that the FBI and the Justice Department’s national security division have been seeking since July to determine how broad a network of accomplices Russia enlisted in attempting to influence the 2016 presidential election." Ergo, I think we know that the application was at least made in July. Conceivably it was granted in August. I'm not sure how long a period there can be between the two.
There are more details, and you can read them at Talking Points Memo.