I guess, it would be important to define "well" first, and it also depends on the type (elevation, flooding frequency, etc.) of saltmarsh you want to gain, as well as the freshwater marsh "source".

My guess is- quickly if you have the desired species present and can maintain water levels without flood or drought. However, your question is site specific.

Some of the factors you'd need to consider to answer that is the inflow an outflow volumes and salinities, obviously, and their capacity for mixing, which may involve clearing surface water pathways and factor geologic sediment profiles. There have been successful restorations of this sort along San Francisco Bay.

Manual introduction of salt marsh plant species may be necessary if there is no abundant source. In the northern Gulf of Mexico, conversion of fresh marshes to salt marsh has occurred well (often irreversibly) within a few years time. Conversion of cypress forests have converted to salt marsh within a decade with salt water introduction.

Water level manipulation has had conflicting results, with largest problems being the ability to adjust structures regularly to suit needs.

Sources in the public domain you may find helpful are:

http://el.erdc.usace.army.mil

http://www.habitat.noaa.gov/

http://lacoast.gov/ (as a source of for 37 Hydrologic Restoration projects)