It took me 15hrs to beat it. It takes the majority of that to get to the second disc. Disc 2 is VERY short.
I beat Assignment Ada today, it was pretty easy once you get inside the building. The first part of it and the very last part of it are the only really difficult parts. You have to fight Krauser at the end
I unlocked Ada in Mercs, but I'm not sure how well I'll be able to unlock the rest. I don't have patience for it I guess. It's fun for a bit, but I do awesome in the dock level killing like 20 people in a row and racking up an awesome score when the huge ass chainsaw guy comes and cuts my head off without hesitation. My shotgun bullets don't even phase him. My friend has an action replay, I'll probably just use it :shrug: