I only tested this during prime time as the drop rate during normal time isn't high enough to warrant the extra effort in confirming the pattern for that time. Also just note that I only did a ~30 mins test, so the pattern may not be entirely accurate, and could possibly change on a daily basis (but probably not as I don't think the devs would've bothered). So here it is. From what I've gathered in 30 mins, there is a 55 seconds interval between each drop period, the only time when letters will drop, and each drop period lasts roughly 5-7 seconds. Let's take a look at an example.
Event starts at 18:00:00 (hour:min:sec)
at 18:00:55 - 18:01:00 is the first drop period
at 18:01:50 - 18:01:55 is the second drop period
at 18:02:45 - 18:02:50 is the third...and so on
However, some numbers I collected fell a few seconds outside of each drop period, so I think it is also possible that the pattern could be like this example.
Okay, so why does knowing the pattern matter? Well, if you can aoe mobs, you can maximize your letter drops by luring mob trains and aoeing them during the 5-7 seconds drop period. And once again for us single target DDs we're left in the dust because the items weren't programmed to drop randomly
I only tested this during prime time as the drop rate during normal time isn't high enough to warrant the extra effort in confirming the pattern for that time. Also just note that I only did a ~30 mins test, so the pattern may not be entirely accurate, and could possibly change on a daily basis (but probably not as I don't think the devs would've bothered). So here it is. From what I've gathered in 30 mins, there is a 55 seconds interval between each drop period, the only time when letters will drop, and each drop period lasts roughly 5-7 seconds. Let's take a look at an example.
Event starts at 18:00:00 (hour:min:sec)
at 18:00:55 - 18:01:00 is the first drop period
at 18:01:50 - 18:01:55 is the second drop period
at 18:02:45 - 18:02:50 is the third...and so on
However, some numbers I collected fell a few seconds outside of each drop period, so I think it is also possible that the pattern could be like this example.
Okay, so why does knowing the pattern matter? Well, if you can aoe mobs, you can maximize your letter drops by luring mob trains and aoeing them during the 5-7 seconds drop period. And once again for us single target DDs we're left in the dust
You are in fact right, the actual method used for event drops defines a list of drops, ordered by position and establishing a max for each letter in a given period (normal period is around 6 seconds).
For instance, given that there are 14 letters, the list establishes that every 6 seconds, all server mobs will drop 2-3 letters of each type. If in those 6 seconds no mobs are killed, or there is not enough mob deaths to drop all letters in that list, it resets and starts all over again. If, on the contrary, enough mobs are killed, the list does not reset until the 6 second loop starts again, so that's why you get 0 letter drops, because the loop is not finished and every letter has already been dropped.
You can also set hours in which these loops are shortened to 3 seconds, or the list is increased and, instead of dropping 2-3 letters, it will drop 4-6 letters.