You could probably get the main story done in about 20 hours or less if you put your mind to it (You've got to accrue enough Chaos points, so that inevitably means doing missions/blowing stuff up for a fair while).
To get 100% completion you'll take anything up to 100 hours or more. I've spent anywhere between 60 and 90 hours on different playthroughs and only got to around 65% completion since there are so many things to collect about the place and blow up (although grinding away for 100% doesn't interest me). The combination of missions, collectables, destructibles and races means you'll be spending a long time roaming around the map for them.
I did a fair amount of destroying/collecting between missions to make the missions "last" as long as possible. While the game remains good while you're just running around blasting stuff, it does lose a little bit of its shine if its the only thing you're doing.
There are only 7 or 8 "Story" missions in total (Can't remember exactly which). So, Story alone, the <20 hours mark would probably be accurate.
I can't even fathom how long a 100% run would take. Story/side quests (Including challenges)/achievements is feasible, but there's SOOO much to destroy and collect it would take a LONG time. I'm just past 70% complete and have ~64 hours logged into a single save. Getting ALL Achievements would take quite some time, as there IS an Achievement for 75% completion.
And besides that, once you get down to that final percentage, scouring the map would be neigh impossible to try and track down everything. At least, without using something like the Map Viewer that shows where any remaining collectibles may lie. --- PC: http://i305.photobucket.com/albums/nn233/cody4783/cody4783.jpg