Apparently Yahoo was hacked yet again with up to a billion user accounts being compromised. The article says Yahoo uses MD5 for password hashing.   

Are the hackers likely to be able to crack the passwords too?
How long will it take to crack 1 password?
Is the time to crack 1 billion  , just 1B * t ?

Yes, they were likely able to crack many of the passwords in a short time.

From the official Yahoo statement:


  For potentially affected accounts, the stolen user account information may have included names, email addresses, telephone numbers, dates of birth, hashed passwords (using MD5) and, in some cases, encrypted or unencrypted security questions and answers.


MD5 is a disputable choice for password hashing because its speed makes cracking MD5-hashed passwords really fast. Also, they are likely not salted, since Yahoo would have certainly let us know. (A salt would have helped to prevent the use of rainbow tables while cracking.)

You can see the drawbacks of simple MD5 hashing when you compare it with the Ashley Madison breach in 2015 which leaked 36 million accounts. In that case, they used bcrypt with 212 key expansion rounds as opposed to Yahoo's plain MD5 which is why back then researchers could only decipher 4,000 passwords in a first attempt.

From the article:


  In Pierce's case, bcrypt limited the speed of his four-GPU cracking rig to a paltry 156 guesses per second.
  [...]
  Unlike the extremely slow and computationally demanding bcrypt, MD5, SHA1, and a raft of other hashing algorithms were designed to place a minimum of strain on light-weight hardware. That's good for manufacturers of routers, say, and it's even better for crackers. Had Ashley Madison used MD5, for instance, Pierce's server could have completed 11 million1 guesses per second, a speed that would have allowed him to test all 36 million password hashes in 3.7 years if they were salted and just three seconds if they were unsalted (many sites still do not salt hashes). 


So, cracking a large portion of the Yahoo passwords is a matter of seconds (while some stronger passwords will remain unbroken). An exact answer would depend on the available computation power and the password security awareness of Yahoo customers.



1As @grc has noted, 11 million hashes per second appears rather slow. @Morgoroth's linked 8x Nvidia GTX 1080 Hashcat benchmark (200.3 GH/s for MD5 total) is a good resource for more up-to-date measurements.
