You are correct. I had assumed a 1 second next-spell-cast delay, but testing indicated that that was quite wrong.
First, basic testing agrees with Yinnyth. I was able to consistently get 12 threnodies per minute once I got the recast timing down (with only occasional errors). So next I tried to figure out why.
FRAPS analysis of casting time.
I don't have a perfect brd gear set; missing 6% in cast time, and don't have Minstrel's Ring. End up at 40% total.
Changed subjob to /rdm for the extra fast cast to see how it would affect things. That puts me at 55% fast cast.
After adjusting to the slightly different timing, I was able to get 14 spells cast per minute.
14 spells per minute is 4.28 seconds per spell. Can figure the 0.28 is human error, in that I can't cast the next spell at the perfect recast moment, and it's a 4 second recast on a 1 second spellcasting time (after fast cast is applied). That's a 3 second casting delay.
To test this effect, I retried using Minuets. Nominal cast time is 8 seconds.
With /rdm and fast cast gear, time between start of Min4 and Min5 is 7.233 seconds. Actual cast time should have been 3.6 seconds. Difference is 3.6 seconds; with allowance for human error, we still get the 3 second delay.
Tried again with /whm and no fast cast. Time between start of Min4 and Min5 is 11.367 seconds. Subtracting the nominal cast time leaves a 3.367 second window, for 3 seconds plus human error.
Overall conclusion: Minimum delay after the completion of one spell before you can cast another is 3 seconds. Using the fastest casting spells available, plus accounting for human error, lowest realistic time per spell cast is 4 seconds, allowing for 15 spells cast per minute.