If we trace the dialogues of the characters from the start of The Deathly Hallows up to the chapter A Place to Hide (in the shabby cafe near Tottenham Court Road), there were many instances where the characters mentioned Voldemort but they weren't found immediately by Death Eaters. To cite:
Before the Battle of the Seven Potters:
“You claim,” said Uncle Vernon, starting to pace yet again, “that this
“—Voldemort,” said Harry impatiently
In the dialogue between Harry and Vernon, Harry mentions Voldemort at least twice. Hestia, one of the OoTP members assigned to escort the Dursleys mentions him once.
In Ted Tonks' home (after the Battle of the Seven Potters):
Harry sat up too quickly: Lights popped in front of his eyes and he
felt sick and giddy. “Voldemort—”
“Easy, now,” said Ted Tonks
At the Burrow:
Voldemort's name is mentioned many times during the main characters' stay at the Burrow.
"..I don’t know what happened to anyone else, four of them chased us,
it was all we could do to get away, and then Voldemort caught up with
(Harry to Molly)
“We’ve been betrayed. Voldemort knew that you were being moved tonight
and the only people who could have told him were directly involved in
the plan. You might have been an impostor.”
“None of the Order would have told Voldemort we were moving tonight,”
“Interesting theory,” said Harry. “Has anyone ever tried sticking a
sword in Voldemort?..."
(Conversation with Scrimgeour, The Will of Albus Dumbledore)
“We know what’s going on! Voldemort’s taken over the Ministry,
what else do we need to know?”
(Hermione in the shabby cafe in Tottenham Court Road, A Place to Hide)
Within minutes after Hermione delivers the dialogue above, Dolohov and Rowle find the trio.
Therefore, it's safe to assume that the Taboo was implemented immediately after the Death Eaters seized control of the Ministry or after they killed Scrimgeour (at the end of the chapter The Wedding).
But then again, if you look at the instances above, Voldemort's name was mentioned in places with very powerful protective enchantments (i.e. 4 Privet Drive, the Burrow, Tonks' home). Thus, it's also possible that the Taboo was already in place, but that it may not have been powerful enough to break the enchantments at those locations.