o_0 That is a very good question...
It was about a month from unlocking the first achievement to unlocking the last achievement but as far as that goes, i slowed right down towards the end going days without playing (silver medals are not fun). I'm gonna say it was around the 30-50 hour mark.
Thing is it could vary so much, depending on your ability to unlock the S ranks and how long it takes you to collect the medals (text guides/no guides/official guide with pics etc.)
Im a solo musician trying to be heard in the world, take a listen to my Rock/Post Grunge style songs, you might find something you like, add me and thanks for your support:
Most proud accomplishments:
Arcade: Joust Defender Super Contra Hexic 1+2 TRON. All @200/200.
Retails: Guitar Hero 2/3/A/WT Kameo (50/50) Dead or Alive 4 Sonic the Hedgehog. All @1000/1000 +0s.