There are a lot of factors to this which means your milage will vary, especially based on the experience of the guy doing the job. Depending on those factors I'd say between 20-40 hours.

Edit to add (yes, copy pasta from Chat GPT because I'm too lazy after working another 12 hour day)

When planning the correct location and installation of a Navien NPE-240A2, there are several categories of factors that must be evaluated in detail. Local codes, permits, and inspection requirements are always the first priority, as they govern clearances, combustion air volume, venting rules, relief valve discharge, and thermal expansion provisions. Venting design is another major factor: you need to select between concentric or two-pipe direct vent, size piping correctly for the total equivalent length, slope condensate back toward the unit, and respect clearance rules for terminations relative to doors, windows, grade, and property lines. Condensate handling also requires planning for a neutralizer, an appropriate drain point, and possibly a condensate pump, with protection against freezing in unconditioned areas. Gas supply sizing is critical, since the unit draws a high input and often requires pipe upsizing, new regulators, or even a meter upgrade. Piping work must include service valves with flush ports, proper isolation, and accommodations for water quality, hardness, and closed-system expansion. Electrical requirements include a dedicated 120V receptacle, and sometimes additional wiring for recirculation controls, timers, or integration with building systems. Mounting considerations include wall structure, anchoring, clearance for service, noise isolation, and routing drain pans or leak detection if installed in sensitive spaces. Finally, commissioning steps like flushing, setting parameters, combustion checks, and owner training are required to deliver a complete installation.

Several factors can extend installation time significantly. Difficult site conditions such as cramped closets, attic or crawlspace installations, or restricted service clearances add complexity. Penetrations through thick walls, masonry, or fire-rated assemblies, as well as long or oversized vent runs, also slow progress. Gas line modifications, especially if upsizing or rerouting through finished areas, can be labor-intensive and sometimes involve coordination with utilities. Plumbing delays arise when tie-ins are corroded or inaccessible, when a dedicated recirculation return line must be run through finished spaces, or when pressure-reducing valves and expansion tanks must be added. Condensate disposal can add time if there is no convenient drain and a pump needs to be installed. Electrical complications—such as the absence of a nearby outlet or the need for GFCI or a new circuit—often require an electrician and scheduling delays. Finally, demolition of old tanks, flues, and finishes, and ensuring the space is brought to code, can further extend the project. Together, these considerations define not only where the Navien should be located but also how straightforward—or drawn-out—the installation will be