As one of the triple constraint of project management, project completion time is very important issue. This is considerable for wide range of projects, especially for the time-terminated ones. Beside standard network planning methods, such as CPM, PERT, PDM etc. there are variety of alternative methods, based on previous ones, which are used in order to obtain even better project duration evaluations. In these methods researchers includes simulation, fuzzy theory, genetic algorithms etc. This paper describes the approach in modification of shortest path search algorithms and their usage in project duration assessment. Presented modification of shortest path algorithms is focused on its conversion to finding longest path in the network diagram i.e. to finding critical path of the project.