Add together the most optimistic and most pessimistic time frames. For example, the company may assess the most optimistic time frame to be one week and the most pessimistic one to be five weeks. The sum would be six weeks.
Multiply the likely time frame by the number "4." For example, the company may assess that the project is most likely to be completed in three weeks. In this case, you would multiply 3 times 4 with 12 as the result.
Add the resultant figures from step 1 and 2. Here, these would be 6 weeks plus 12 weeks equal to 18 weeks. Divide 18 weeks by 6. You would get 3 weeks as the answer.
Repeat the same process for each sub-process. Some processes may be interdependent; that is the start or finish of a step depends on the preceding or succeeding steps. Some processes may be accomplished independent of one another. Therefore, the company needs to take all these into account before estimating the total completion time for the project.