You can look up the project units for a specific unit type, Length in the code below, and use that to determine the conversion needed.
import clr
clr.AddReference('RevitAPI')
from Autodesk.Revit.DB import *
clr.AddReference('RevitServices')
import RevitServices
from RevitServices.Persistence import DocumentManager
from RevitServices.Transactions import TransactionManager
doc = DocumentManager.Instance.CurrentDBDocument
unitType = UnitType.UT_Length
currentDisplayUnits = doc.GetUnits().GetFormatOptions(unitType).DisplayUnits
OUT = currentDisplayUnits