I couldn't find a specific timeframe, but Sightseeing Lore says it was the Syndicate's first priority. So, with their money and resources, I imagine the project was complete within a few months to half a year? Maybe? I'm not sure how long it takes to build a bridge.
Quote:Sightseeing #20 - Highbridge
After the Calamity rent eastern Thanalan asunder, one of the first projects commissioned by the Syndicate was the construction of a bridge to span the newly formed chasm and restore the trade route with lands transpontine. The city which has sprung up along its edges is populated almost solely by scholars investigating the nearby ruins of the Invisible City.
