Let's say the timetable is 18 weeks and the aggregate for all jobs is 30 weeks. The average of the two is 24 weeks, and my experience tells me 24 weeks will be a more realistic timeframe. In other words, the project will take 6 weeks longer than suggested by the timetable . Do try to calculate the time for each unit as accurately as possible. If you don't know, find someone who does such as a consultant.
Vendors can be helpful though likely optimistic.
At the same time, make sure that they can start on schedule and that they can allocate the proper resources for the duration. Editorial contributions and ideas for the Project-Manager site are encouraged from practitioners and experts, in all areas of project management.