Stol'd from Wiki:
Megaman Legends - August 31, 1998
The Misadventures of Tron Bonne - May 5, 2000
Megaman Legends 2 - October 24, 2000
Time since last game: ~9 years.
Megaman ZX - September 12, 2006
Megaman ZX Advent - July 12, 2007
Time since last game: ~2 years.
Also, to cover my ass, all listed dates are for the games' release dates in the NTSC regions.
If anyone else wants to add something funny to this, they can name how many times the ZX series characters got a cameo vs the Legends series characters. Either way, it looks like we'll be seeing more ZX sooner than Legends. Just be patient.