How much of the oil will stay in french fries?
There's too many factors to have a set percentage of oil that will stay, but lets cover a few common things that determine the oil in your final product.

Heat of the oil:


Your oil needs to be plenty hot enough to actually fry in.  For fries, you'll typically want to shoot for between 350-375 F.  If you don't have a thermometer, then get one!  If you really don't want to get one, a white bread cube will fry brown in about 60 seconds at the right temp - but a thermometer is best.  If the oil isn't hot enough to fry in, it will start to absorb the oil.  So before you ever fry anything, get that oil up to temp (and make sure its that way between batches!).


Coating of the fry (or other fried food):


A batter coating is just going to hold more oil than a plain french fry.  If you're trying to be health conscious about french fries (but then again, why are you eating french fries!?!), avoid batter coating them and go with a 'plain' fry.


Post frying treatment:


Shake, shake, shake!  Give the basket a good vigorous shake.  Don't have a basket?  Use a strainer or at least a cooling rack with a mesh-type grate.  You're trying to physically shake as much oil off the fry as you can.  Additionally, I like to lay them on paper towels and then fold the paper towels on top.  See all that oil? Simple math, if its on the paper towel, its not in the fry anymore.


Crowding the pan:


If you crowd the pan you fry in, you'll lower the temperature of the oil and give the oil not enough circulation.  This means longer to fry and more time not at the right temperature.  This means more oil in the final product.  Its better to do some smaller batches than to try to cram it all in one batch.


The right oil:


The main thing here is that you need an oil that can fry hot enough without smoking.  This goes back to the right temperature.  (In case it hadn't sunk in yet, temperature is important!)  If you can't get it hot enough, then you're going to absorb a lot of oil.  Olive oil is a bad choice.  I usually use peanut or canola oil.