I have a few small mounted orchids - it's very dry here too. Some I have had to add more sphag to a few months after mounting the plant. In summer they get watered well at least daily - in extremely hot dry weather, water twice a day.
Plants I have mounted - Phal. deliciosa, Phal. violacea, Aerangis biloba, Neo. falcata, Bulb. lasiochilum, Den. bigibbum, Tolu. sylvestris. Except for the Bulbo, which I have only had about 6 months, I have had these plants mounted about a year (Tolu about a year and a half). The Phals and Aerangis were mounted after I had them a few months potted and going downhill. The Neo I have had a bit less than a year. All seem to be fairly happy, tho the Phals and Aerangis were rootless when I mounted them. They have grown new roots, tho not a lot. They seem to only grow during warmer months, and slowly at that. Haven't bloomed any yet. I know some people even add sphag for the hot summer, and take some off again in cooler weather.
Hope that helps