I know that when someone comes here on a K-3 visa they have to file for adjustment of status. But how long does it take after you file for it?? Does the person get a green card when they enter into the U.S. on K-3 visa ?

See my response to your other thread. Green card is received after a successful AOS, which can take months to get approved. CR1/IR1 petitioned for using an I130 only will get that green card right away. It is the better visa and worth considering instead of a K3.

K-3 Is a NON-Immigrant visa and will not get green-card upon entry to the USA, that is what adjustment of status is all about (Filing for Green-Card). AOS can take as little as 4 months but depending on processing can take YEARS to complete. K-3 holders are also not work authorized and need to get EAD to work, this take up to 90 days or more to get.

CR-1 visa is an IMMIGRANT visa and gets a green card upon entry, no AOS ($1010) or EAD required.

It is recommended to compare VJ timelines for CR-1 and K-3 for your country and see how much longer one takes over the other. In most cases CR-1 takes a few weeks to a few months longer to process, and has the BIG benefit of not having to adjust status.