Biggest we've done is around 300 meters, with out checking each bit you can never be too sure of how bad it may be!
We charged £1 per meter and then added £50 on just to be safe

We had a gutter vac and it only took a couple of hours in the end luckily but could have been far far worse!!!
Then again we did another gutter clean on a 4 bed detached just a straight forward 2 high level runs(1 front and 1 back) and one lower run half way across front charged £30, should of taken 15/20 mins with the Vac ended up the roof tiles had moved forward and even the thin vac pipe wouldnt fit and had to use a ladder to move tiles and help guide vac pipe, MISSION!!!! Took about 45 mins in the end at least!
Id say go in slightly heavy as like you say if they are all full, it WILL be a right pig to do!
Good luck