The Harbor bridge structure will most likely be structurally similar to the concrete bridge spanning Disneyland Dr for DTD. These structures usually take 18 to 24 months and do not need to have long closers for an active road underneath. Maybe some overnight closings to install and remove falsework, but closures could be reduced to only overnight closures. I doubt any midspan supports are needed, which will help the timeline process and reduce the need for any large alterations to Harbor below. I would take a blind guess of an 18 month timeframe of construction. This project would need to be in conjunction with the city's planners, engineers and design, which can take some time, but I'd hope these plans are already completed, hopefully. My guess it would probably take the same timeframe of the parking structure and the bridge structure spanning the parking structure to the intersection of Magic Way and S Anaheim Blvd.
All speculation on my part.