Wrong answers: two weeks, or "unlimited" (because everyone knows there will be peer pressure not to use vacation time).
Right answer: four weeks, use it or lose it (as in, we want you to check out for a proper vacay a few times a year).
The definition of vacation = I am going to be on a beach/wherever, not checking email, probably drinking, and definitely not thinking about work.
Location is also important. If you have a 2-3 hour round trip commute each day, you'll hate your job no matter how interesting the work is.
A "real" unlimited vacation policy is a strong positive signal about the company. It shows that (a) people are trusted to manage their own time and not to abuse the system and (b) the people who work there are good enough and driven enough to deserve that trust.
"Unlimited" holidays is a scam, unless they'll put in writing that you're entitled to 6 months holiday per year.