Also worth noting that epoxy is not a generic product. There are many different types, and the better grade products are formulated to provide specific working and cured properties.
Twin tube injector convenience epoxy from the hardware store will not provide the same level of cured properties as a marine or aerospace formula for example.
Some high grade epoxies are formulated as adhesives; others are formulated as penetrants or surface coatings, etc.
The better the epoxy you use, the longer the fingerboard coating will likely last. Proper metering and mixing is also critical.
Also, nut height and action play a role. The higher the nut slots and action, the more effort you have to put into playing, and the quicker a board coating will likely wear. The epoxied fretless I play the most has low nut slots and action and is strung with older Thomastik flats. The board has worn very little to date and has a nice growl.
If I was doing a thickly built epoxy coating, I'd still apply a low viscosity penetrating coat to begin with.
If I was using an epoxy product for the first time, I'd do one or two small batch test mixes on scrap before committing to the actual workpiece to get a feel for how it handles and cures.