With excessive duct lengths or runs with many turns, the "back pressure" of the duct is too high. This cuts down on the amount of air that is extracted from the dryer. We often hear of 50 foot duct runs and 2+ hour dry times! 

Typical residential dryers are rated at 160 cfm. However, boosting is generally needed in order to maintain a minimum airflow of 100 cfm as duct length increases and bends are introduced in the duct line. According to Fantech (based on surveying the recommendations from dryer manufacturers, and local building codes in selected areas) dryer boosting is typically required when the duct length exceeds the following:

Maximum duct length with 
0 bends = 25'
1 bend = 20'
2 bends = 15'
3 bends = 10'