The only work of which the results are 100 percent guaranteed is working for your enlightenment. If you are sincerely working for this, the result is 100 percent guaranteed. Note that when it would happen is not mentioned here, but that the result is guaranteed.
The answer I have created for the following question will further explain this approach: Can you give me a timetable for 24 hours? How can I manage my work, eat, and sleeping?