In Harry Potter and the Goblet of Fire,
Barty Crouch Jr. pretends to be Alastor Moody for most of the school year, with the help of copious amounts of Polyjuice potion.
How much Polyjuice potion did he need?
Based on a reading of the text (see below), the amount he used might actually be a lot smaller than we would immediately imagine, perhaps less than 1000 x 1-hour doses.
Assuming each "swig" is one 50ml dose, he'd need just 50 litres of Polyjuice potion in total. If he was brewing it in a 5 liter; Standard 'Size 2' Cauldron, then he would need to brew just one batch per month for 10 months.
We only see him on one weekend day, and that's at an off-site meeting of the Hogwarts senior staff in Hogsmeade (an opportunity he would have no doubt used to get hold of Polyjuice supplies).
There is no canonical example of a DADA lesson ever being taught on a Tuesday
Assuming he teaches two periods (6 hours per day), that leaves him with 18 hours to spend in his study with the door locked.
He makes a point of stating that he only eats his own food and drink. That excludes him from attending mealtimes aside from on special occasions.
Moody had told them all during their last Defence Against the Dark Arts lesson that he preferred to prepare his own food and drink at all times, as it was so easy for Dark wizards to poison an unattended cup.
He's known to be a late riser and schedules his classes accordingly
‘D’you think it’s too early to go and see Professor Moody?’ Hermione said, as they went down the spiral staircase.
‘Yes,’ said Harry. ‘He’d probably blast us through the door if we wake him at the crack of dawn, he’ll think we’re trying to attack him while he’s asleep. Let’s give it ’til break.’
He sips on his flask whenever he's expected to go anywhere unexpected, strongly suggesting that he's only taking the minimum amount required to get by
When the bell finally rang, they hurried out into the corridors towards the Dark Arts classroom, and found Professor Moody leaving it.
He looked as tired as they felt. The eyelid of his normal eye was drooping, giving his face an even more lop-sided appearance than usual.
Taking all of that into account, he might be getting away with taking less than 4-6 doses per day on teaching days and none at all on at least 2-3 days per week. Additionally, he's probably stealing at least 2-3 hours a week by starting his morning session quite late and taking a lunch in his room.
600 large doses
According to HP Lexicon, Barty Crouch Jr. began impersonating Alastor Moody on August 31 and was caught on June 24th, 298 days later. Assuming that he remained as Moody 24/7, that would make him need over 7,000 hours worth.
The dosage for a single hour is described as a "large dollop".
Three glass tumblers stood ready on the toilet seat. [...]
Once we’ve drunk it, we’ll have exactly an hour before we change back into ourselves.’ [...]
Hermione ladled large dollops of the Potion into each of the glasses. (CS)
Polyjuice potion works for up to twelve hours at a time.
The effect of the potion is only temporary, and [...] may last anything from between ten minutes and twelve hours. (Pottermore)
So at a minimum he would have had to take about 600 large doses.
I'm assuming that he would have remained Moody constantly based on A) The effects of him transferring back at the end of GF seem to imply that it wasn't something that happened often. B)Dumbledore's line about how careful Barty Crouch Jr was.
Around 4800 doses
10 months, 16 doses a day (10*30*16 = 4800 doses). Or around 3400 if we exclude weekends (4800*5/7).
According to the Chamber of Secrets, chapter 12, "Harry drank the potion down in two large gulps." I don't know how much a "large gulp" means, but if it is about 50ml, then you would need 480 liters of polyjuice -- or 340, if we exclude weekends.