Well, maybe your server is really slow, but as a dps, I never wait more than 20 minutes to join a normal diadem in solo via DF, this can be even shorter as a heal.
The only advice I can give you is either planning with your fc for a gatherer group via your airship or just trying your luck during hours when there is more players (between 5 to 10 pm, diadem procs really fast).
Either way it's just a matter of patience.