The underlying of the Ice Age option is the average block time calculated as:
where is the current block timestamp, is the timestamp of the block from which the option counts the average and is the number of blocks since the counting started.
Each individual block can have any time between above 1 second as the PoW algorithm can generate blocks with times randomly distributed around the target block time of 15 seconds (but not shorter than 1 second). The fluctuation in the block times depend on one of the few factors:
changes in hashrate can cause temporary block times variations
miners leaving the network would cause the times to expand slightly
miners joining the network would cause the times to shrink slightly
Ice Age mechanism in Ethereum may cause the block times growth
When buying or selling an Ice Age option you are taking a view on one or all of the above. You may expect a massive shift in the hashrate or you may expect the Ice Age prevention mechanisms to fail.
Historical block time chart (see etherscan.io for the current values):