https://wiki.torn.com/wiki/Faction#Chaining
On this page, it says that a chain cooldown is 10 seconds per hit.
- Once a chain breaks, it will enter its cooldown state, which lasts 10 seconds for every hit made in the chain (e.g. 100 seconds for a 10-hit chain).
https://wiki.torn.com/wiki/Chain
However this page says 6 seconds per hit.
If your chain breaks (i.e. no one in your faction makes a hit for 5 minutes) it will begin to cooldown at a rate of 6 seconds per hit made. So if you reach a chain of 100, it will take 600 seconds to reach zero and be fully 'cooled down'. You may attack during the cooldown period, however it will have no effect on the cooling chain.