We see a fair number of wands in Ollivander's shop. When Harry visits to be chosen by his first wand, he sees "thousands of narrow boxes". In another answer it is estimated that Ollivander sells about 160 wands each year. It appears he has a great surplus in his shop.
How long does it take Ollivander to make a wand? Or, how many wands can he make in a year?
Ollivander makes Luna Lovegood a new wand in a matter of weeks in Deathly Hallows.
Mr Ollivander had sent Luna a new wand that morning. She was out on the back lawn at that moment, testing its capabilities in the late afternoon sun.
(Deathly Hallows, Chapter 26, Gringotts).
This occurs the day before the raid on Gringotts. Based on the dates which JKR uses in the documentary J.K. Rowling: A Year in the Life, the Battle of Hogwarts took place on 2nd May 1998. This is backed up by her tweets - she has taken to tweeting on 2nd May each year to apologise for killing off characters in the battle. Using this timeline, the Gringotts break-in took place on 1st May and Luna received her new wand on 30th April.
Ollivander can't have started making the wand until after he left Shell Cottage. Unfortunately, we don't know when exactly this was. However, it can't have taken him more than a month. We know that it was already April by the time he left.
"I'm sorry," he told Fleur, one blustery April evening as he helped her prepare dinner. "I never meant for you to have to deal with all this."
(Deathly Hallows, Chapter 25, The Wandmaker).
So making her wand could've taken him a couple of hours or it could've taken him several weeks. But it took him no more than one month.
I would expect him to be able to produce more than one wand a month. As the question says, he owns thousands of wands at any given moment. Given that he sells hundreds of wands every year he must be able to make a dozen or so every month to make sure his supplies don't get depleted. Although, as Jeremy French says, some of the wands may have been on the shelves for a very long time.
According to JKR's Twitter, as quoted in this article, The Battle of Hogwarts took place on 2nd May, which means that the Gringotts break-in was on the morning of 1st May. Luna is shown playing with a new wand the day before:
Mr Ollivander had sent Luna a new wand that morning. She was out on the back lawn at that moment, testing its capabilities in the late afternoon sun.
Harry Potter and the Deathly Hallows - p.420 - Bloomsbury - Chapter 26, Gringotts
So that's the 30th April.
Mr Ollivander is shown returning home just a few pages earlier (p.414) on the very day of Teddy Lupin's birth. Unfortunately, I can't find an exact date but it can only be a few days, weeks at the most. Teddy Lupin is frequently referred to in Google search results as having been born in April, so a weak Ollivander made Luna's wand in less than a month. This is obviously after having been tortured and imprisoned in shocking conditions for over a year and in amongst settling back in at home. So I'd say at full capacity, the answer to your question is: not that long. It's hard to get the upper bound to be more precise than a month, but I should have thought it was a lot less than that especially as, as your question points to, he probably needs to average 3 a week just to keep up with demand.
