According to STOWiki's entry on Lock Boxes:
This does seem to show a trend of a new Lock Box every 3-4 months, and looking through the release notes around those dates, I could not find any correlation between changing the current lock box and major content updates, with the exception of the Tholian Lock Box, which was released 7 days after the Season 6 update (Jul 12 2012).
The other established Cryptic/Perfect World title that uses lock boxes/keys, Champions Online, I am not as familiar with - and attempts to glean similarly useful information off of their Community Wiki proved to be impossible (At best I can tell you they have at least two different lock boxes, and they are opened with Z-store purchased keys). And Neverwinter is too new to base any lock box release schedule on.
So, I'd say roughly 3-4 months is a fair bet - We'll see, as we're coming up on 4 months, and there might be a new lock box to go with the upcoming Legacy of Romulus update.
As for the worth of the lock boxes? Last I looked;
- Cardassians go for around 100,000+EC/unit
- Ferengi are around 10,000EC/unit
- Tholian and Temporal seem to bounce between 20,000 and 25,000 EC per unit for individual listings, but if you change the Exchange sorting to "price per unit" groups of 20 go for 17,000EC/unit
- Dominion boxes, being the current box, are the cheapest, usually going for under 10,000EC/unit - although the price fluctuates
Prices often do go up after the box is no longer available (Note; there has been at least one event wherein all the past lock boxes could drop for the duration of said event). The Cardassian boxes, being the oldest and in the shortest supply, seems to prove this by being the most expensive. But then The Ferengi boxes, being "out of print" the second longest, defies it by competing with the current box for the cheapest.
I'd guess that part of what influences prices is the desirability of what they (potentially) contain. Drop rates during the box's tenure might also play a part, as I do seem to recall enemy ships literally firing Ferengi Lock Boxes at me (They were a potential drop from mines and targetable torpedoes.)