These seem to be too expensive for what you get and has scared most clannies off. Would be nice to see this changed somehow, whether that be extending the term or lowering the cost it seems a shame that no one uses these. It gives a little incentive to value plat for more active clans and should free up some resources for people without alts or on a blank return.

How long does the chest remain?

30 days for 10k plat according to Dank.

The 3 clan chests have been increased from 30 days to 60 days.
Plat price unchanged.

60 days... 10k plat... 6 or 8 hour spawn times on those chests. Not worth wasting plat on it. If the chests were 30 days, cost 5k plat and had maybe 2 or 3 hour respawn time, it would maybe be worth it. Or if you have a clan all for your self with no other members in it, that 60 days and 10k plat chests that uses 6 or so hours to respawn be good.

60 days * 4/day = 240 gpots (or what ever the chest gives) @ (say) 2,500 gold per gpot = 60 gold per plat valuation for a 10,000 plat chest.
This assumes a perfectly efficient clan that exploits the chests at their maximum.
An inefficient clan that only take the chests once per day only values plat at 15 gold/plat.
I thought the old rate used to be 10 gold/plat (but that was a very long time ago).

Maybe my maths or the valuations are wrong or out of date.

It's probably above 4k since castle quests cost that much to repeat, but there is always a chance those are taken. All that is supposing you can get them with gold at all from people.