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.
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 settings or exit the
sketch.