Infinite Lines
You cannot convert an infinite length line
to a finite length line. However you can convert a finite length line
to infinite length, and clear Infinite length to convert it back to finite
length.
To create lines of infinite length:
Click Line
(Sketch toolbar) or click Tools,
Sketch Entities, Line.
In the Insert Line
PropertyManager:
Under Orientation,
select an orientation.
Under Options,
select Infinite length.
Click to sketch one
or more lines.
In the Line Properties PropertyManager,
do any of the following:
Under Options,
select For construction.
Under Parameters,
set a value for Angle 
(if you selected As sketched
or Angle as Orientation).
Click 
to return to the Insert
Line PropertyManager where you can change Orientation,
Options, or Parameters,
or exit the sketch.
Related Topics
3D Line PropertyManager
Insert 3D Line PropertyManager
Insert Line PropertyManager
Line PropertyManager