To directly answer the question...
1ft = 0.3048
1in = 0.0254
As a note to others reading this thread batFINGER's answer includes space_view3d_panel_measure module which is solely for printing out units in an easy human readable format and is not needed for unit conversion. His script also includes 'scale' which is the scale value set in propertys/scene/units/scale. Unless you are using a scale other than 1 (default) it to is unnecessary although it would be good if your script will be used by other people in which case you never know if someone is messing with scale settings or not.
The simplest way of doing unit conversion is...
distance = 2+(11/16)
inch = 0.0254
distance *= inch
Here distance is the value in inches we want converted. In this case we did 2+(11/16) to demonstrate how we would include a fraction. Its the same as saying 2" 11/16" in real life. The simple equation "distance *= inch" will convert distance from inches to Blender Units.
Below we can demonstrate a translation by moving the X axis of a selected object by 2" 11/16"
bpy.ops.transform.translate(value=(distance, 0, 0))
A full script example for this is...
import bpy
inch = 0.0254
distance = 2+(11/16)
distance *= inch
bpy.ops.transform.translate(value=(distance, 0, 0))