The birthday paradox. After a few hundred to low thousands of manual captcha's, someone goes Hey... I've seen this one before!. After 2-3 duplicates, they're

The birthday paradox. After a few hundred to low thousands of manual captcha's, someone goes Hey... I've seen this one before!. After 2-3 duplicates, they're pretty sure that the corpus is small. At that point, it's worth writing a script to grab as many captchas as possible. They can then manually analyze them. A back of the envelope calculation shows that with 30 people, they should be able to solve all of the captchas in a working day, even accounting for breaks.