I would probably say about 10-12 hours. I played on normal difficulty and was trying to find all the collectibles and COG tags. I found 20 of 42 collectibles, and six or seven COG tags out of 15 I believe.
EDIT: One of my complaints (only one so far I think) is that all the ammo clips, crates and cases don't refill weapons other than the basic starting weapons (Lancer, Retro Lancer, Gnasher, Sawed-off and the sidearm). I don't think there was any explanation given why it changed, so it kind of irks me.
Maybe its their way of getting people to use more of the weapons. In Gears 1/2, once I got the sniper rifle, I NEVER dropped it.