The whole first 2/3 or so of the game is pretty much "sandbox" mode. You can take missions whenever you want them to happen, and in whatever order. I spend probably just 2 days on tinkering around and building up cash and the base before I even took on more than 2 missions. The last part of the game -does- get a little hectic and linear, but it's fun. I guess I put 2 weeks straight into the game, sometimes more time per day than I really should have :oops: , but I don't regret it one bit - and I even pre-ordered it at full price. -I- think it was worth it.
Hope this helps some