It adjusts every 2016 blocks found. You probably were looking for "about every 3 days" though, as thats what it generally works out to.
The actual timeline changes based on the network hashrate - the point of difficulty is adjusting in order to keep the rate at which blocks are found consistent. The higher the hash rate, the higher the difficulty needed to slow mining down to the algorithmic target.
So if network hashrate tripled overnight, the difficulty adjustment would happen closer to 1 day than 3. If half of everyone left the LTC network overnight, the adjust time could take more than 3 days until 2016 blocks are found and the difficulty lowers.
I can't explain this, but it has seemed interesting to me... There is a very consistent pattern when looking at the most recent 500 blocks before or after a change in difficulty: Network hashrate hits nearly its highest relative point at that time. Said another way, sometimes hash rates spike in the middle point between difficulty changes, but MUCH more often hash rates spike soon before or soon after a difficulty change. I don't know what the reasoning is for this, but I'm guessing it probably reflects the algorithm automated pools use to pick which coin to mine - which results in large fluctuations in hashing power.