This is a speculation form me (and not information from Feerik)
After seeing some dragons here facts:

Dragons appears for (maximum) 2 hours!

There come in order:
Zil Warriors
Sap Heart
Runic Legion
Kotoba
Mercenaries
Nehantists
Dessert Nomads
Noz'Dingard Envoys
Pirates

Between two dragons these are at least 6 and maximum 10 hours (perhaps updates or server time outs increase the time)

Because of that about all 10 hours +/- 2 hours appears a dragon. (But be careful you cann't say in 30h +/- 2 hours appears a dagon)