Umbridge left the Hospital wing and Hogwarts before the end of the school year in Order of the Phoenix:
Professor Umbridge left Hogwarts the day before the end of term. It seemed that she had crept out of the hospital wing during dinnertime, evidently hoping to depart undetected ...
The HP Lexicon says that the Hogwarts Express returns to London in the third week of June, so at the latest probably somewhere around the 21st.
Harry learns about Ollivander's disappearance on his sixteenth birthday (31st of July):
To Mrs. Weasley’s displeasure, Harry’s sixteenth birthday celebrations were marred by grisly tidings brought to the party by Remus Lupin
“Talking of Diagon Alley,” said Mr. Weasley, “looks like Ollivander’s gone too.”
Given the nature of the exchange I'd guess that's very recent, sometime in the last few days (the 29th to 31st July). However, even if Ollivander's departure had happened a week before Harry's birthday - and he simply hadn't been told about it for that time - there's still about a month between Umbridge departing Hogwarts and Ollivander being taken; that's more than enough time to purchase a new wand since that would almost certainly be top of her list of things to do (a witch as concerned with status as Umbridge isn't going to remain without a wand a second longer than necessary).
There's another point that, despite not actually being relevant due to the above, I'll touch on anyway:
From the description of Ollivander's shop when Harry buys his wand at the beginning of Philosopher's Stone there are stacks of boxes reaching to the ceiling; even if Ollivander is no longer around to make new wands there are still plenty of Ollivander made wands to be chosen by. Personally I can't see Umbridge having any qualms about essentially stealing a new wand from somebody who appears to have vanished.