"ADA recommends a 3:12 slope which means for every 3" of vertical rise you are required to have at least one foot of ramp (14.5 degrees incline) Measure the vertical rise from the ground to where the ramp will sit on the vehicle, stairs or threshold. Take that measurement (inches) and divide it by 3. This will determine the length of ramp needed for an unoccupied power chair." Source: http://www.discountramps.com/articles/correct-wheelchair-ramp-length.htm
This measurement is based on ramp for an unoccupied wheelchair.
For a ramp with the occupant in the wheelchair, a 2/12 slope is recommended. "ADA recommends a 2:12 slope which means every 2" of vertical rise requires one foot of ramp (9.5 degrees of incline)" Source: http://www.discountramps.com/articles/correct-wheelchair-ramp-length.htm
From a personal standpoint, ADA recommendations are usually based upon statistical averages. The ramp should be long enough that the person in the wheelchair can comfortably ascend and descend the slope keeping in mind the degree of ascent the wheelchair is capable of and the braking ability of the particular wheelchair. The incline should not be so steep as to make stopping on the slope impossible.