Hiya
not sure if this helps but in the Lakes our assessment areas are usually designed to last anything up to about 3 hours (with a minimum of about 90 mins). We do 2 areas a day and then are expected to pass a minimum of 8 consecutive days of assessments before being fully graded (we dont have a novice status). Our assessments are continuous so at each month's training weekend handlers have an opportunity to take 2 assessment days. As you can see the process to grade can be quite lengthy - particularly when you have a couple of fails part way through the 8 days - as the counter starts again from zero (although we dont perhaps get as much pressure as if we had only one or two opportunities a year to pass).
It's dificult to give an exact answer - but some examples of assessment areas that myself and others did in 2006/7 :
Borrowdale Valley Keswick - Coombe Gill - working the ground south from the start of the Coombe and east of the beck - to the skyline all the way through the crags then underneath Raven Crag to the far southern end where the steep gill runs. (Area took about 2 hours) half an hours rest then the opposite side of Coombe Gill - same sort of thing.
Patterdale - at night (gale force winds and blizzards) - from the Hostel at Greenside Mine (Glennridding) search the area of footpath and a good swathe of land either side - all the way up the Red Tarn path to just before the zigzags then cut North to skirt underneath Catstye Cam to the reservoir - cross the reservoir and back down the track to the hostel - approx 5km
Loweswater - Mosedale Valley - working a line search with another handler under assessment - covering the whole of the valley south from Raven Crag from valley floor to skyline on the eastern side of the beck - high winds and daylight - took approx 3 hours.
Hope that helps a bit - all I know is I just dont try to think about the area size and mentally just chop it into small bits!
Cheers
Kaz