The reset in a castsequence macro starts with the first press of the macro sequence.
Not the most recent press.

It's very easy to test this...

/castsequence reset=5 Spell 1, Spell 2, Spell 2, Spell 2, Spell 2, Spell 2, Spell 2, Spell 2, Spell 2, Spell 2, Etc..
Spell 1 will regularly come up every 5 seconds.

If you really want to know about castsequences, check out http://forums.worldofwarcraft.com/th...81820910&sid=1 .... the 5th reply is the start of the castsequence stuff.