Well that all depends. Are you doing this manually or with a tool such as DTS xChange from PragmaticWorks? If doing it manually then it really all depends on the complexity of the DTS Package and your knowledge of SSIS. If using the tool then they may have an estimator but I haven't seen it take more than a few minutes, then you get to spend a little bit of time cleaning a few things up. If you can provide more details and possibly and example we can help more. Many of us have spent a great deal of time migrating DTS to SSIS.
If you download DTS xChange from pragmatic works, it has a free estimator process that will look into each package and let you know how long the project is going to take you (manual or automated) based on a number of knobs that you turn. It then spits out a report to give in PDF or Excel to your PM.