How long would i take to get all 79 achievements for Halo 3?

[Lil Boy]

My cousin let me borrow his Halo 3 but I'm going to go for all 79. How long would it take? I have a lot of experience playing Halo 3 and beat my cousin a lot at Halo 3 and hes good.

 

[Avery117]

there is only 49 achievements on halo 3 and you can only get them all if you have X-box live and before you beat the game on any other difficulty beat it on legendary because it gives you the achievements for all the difficulties. I have had the game for a while now and I only have around 30 of them so good luck