My Phal hybrid with very waxy blooms lasts 5-6 months.
As far as species, Dendrobium cuthbertsonii is known to bloom for a long time, I think it can easily last 6 months in good conditions
Otherwise ones I can think of that blooms a long time (but not in the way you mean) are Psychopsis. The individual blooms only last 3-4 weeks, but being sequential bloomers and since I have 2 spikes (3rd on the way) on mine there's always at least one of them flowering, or at the worst 2-3 weeks of nothing while buds grow. A spike can continue to bloom for up to 10 years, so if you have multiple spikes, the plant is pretty much never out of bloom!
Of course longevity is not set in stone per species/plant, it really depends on the cultural conditions in the end.
Completely orchid obsessed and loving every minute of it....
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