Is 20 feet too long to run bathroom fan duct for 100cfm fan?
This will completely depend on the number of elbows, the size of the room being ventilated, and the power of the fan.
In general for a bathroom, you'll be looking to have an Air Change per Hour (ACH) of 8. To accomplish this, you'll have to select a fan based on the size of the bathroom, and the equivalent duct length.
Bath room size
The first thing you'll have to figure, is the size of the bathroom.  For this, simply multiply Length (ft.) * Width (ft.)* Height (ft.). This will give you the size of the room in cubic feet.
Example:
Given a room that is 10 feet long, 8 feet wide, and 8 feet tall.
Volume = Length * Width * Height
Volume = 10 ft. * 8 ft. * 8ft.
Volume = 10 * 64
Volume = 640 cu. ft.
Equivalent Duct Length
To calculate the equivalent duct length, you'll have to consider not only the length of the duct. You'll also have to know the duct construction, and number of elbows. The basic rules that apply to both 3&quot; and 4&quot; duct, are as follows.

Measure the length of straight duct.
If the duct is flexible aluminium, multiply the length by 1.25.
OR If the duct is flexible insulated, multiply by 1.5.
For each elbow, add 15 feet.
For each terminal (wall cap, roof jack), add 30 feet.

Example:
Given this duct:

There is 10' of straight smooth walled duct. Plus 2 elbows, and one terminal at the end.
EDL = 10' + (2 * 15') + (1 * 30')
EDL = 10' + 30' + 30'
EDL = 10' + 60'
EDL = 70'
Determine the proper sized fan
Once you've got all the information, it's time to select an appropriately sized fan.  To do this, you can use a chart like this

Using this chart, you'll find that for a 640 cu. ft. bathroom and 70 ft. of duct (equivalent length), you'll need a 110 cfm fan to achive the recommended 8 air changes per hour.