I'd say longer than that. Since Wimbledon really. Since then Slava is 0-10 in 3rd sets.
Thats when the choking began, but she was still playing at a good level after that. She wasn't losing to some random players until after the US Open.
She lost to Lisicki in a good match at the Olympics. She retired against Ula, she lost against Jankovic in a tournament she shouldn't have been at, and she lost to an on form Vinci at the US Open after choking match points. During that time, she wasn't playing as good as Wimbledon but she was playing at a decent level. Certainly better than the level she's currently playing at.
Since then, she has been losing to players who aren't anywhere near her in the rankings.