Equipment that reduces Song Spellcasting Time reduces the casting time of songs like Fast Cast does, and stacks with any Fast Cast equipment or Job Traits.
Normally a song activates at 80%, and you can start singing the next one at 100%, which works out to /wait 12 for most buffs.
With Loquacious Earring or Rostrum Pumps you can start singing the next after /wait 11.
With Sha'ir Manteel, Yigit Gomlek or Fast Cast II from RDM37 subjob you can start the next after /wait 10. Timing is pretty tight with Gomlek; adding Rostrum Pumps or Loquacious Earring helps.
With Minstrel's Ring (-25%) a song activates at 60%, and you can start the next at 80%, or /wait 9.
With Minstrel's Ring and one of Yigit Gomlek, Sha'ir Manteel or Fast Cast II from RDM37 subjob, a song activates at 50%, and you can start the next at 70%, or /wait 8. Again, timing is tight with Gomlek and Pumps or Earring make it more consistent.
With Minstrel's Ring, Fast Cast II, and either Yigit Gomlek or Sha'ir Manteel, you can start the next after /wait 7.
With Nightingale, a song activates at 40%.
With Nightingale, Minstrel's Ring, Yigit Gomlek and Rostrum Pumps, a song activates at 25%.