The script takes grabs the title of the active window and print out the amount of time spent on the output file, after that you can use this data to see how much time you spend on each application.

c_me.py

from AppKit import NSWorkspace
from time import sleep
from time import time
import sys

if len(sys.argv) != 2:
    sys.exit("Usage: SCRIPT output_file.txt")

file = open(sys.argv[1],"a")
lastname = ""
t = time()
while True:
   activeAppName = NSWorkspace.sharedWorkspace().activeApplication()['NSApplicationName']
   if activeAppName != lastname:
       if lastname:
           file.write("\n" + "% 10d" % (int(time() - t),) + "\t" + lastname)
       lastname = activeAppName
       t = time()
       file.flush()

   sleep(1)