The item cache is rebuilt whenever the game client version changes. The game client version changes after a hotfix, or a patch is released by Blizzard. Depending on the frequency of these releases by Blizzard, depends on how often TSM will need to rebuild the item cache.
It may take a few minutes for the process to complete, you can leave the game running while it is in progress. Reloading or logging out will interrupt the process.
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Things to consider after your item cache has been rebuilt
1. Your vendorbuy values will need to be repopulated by visiting the vendor using the TSM interface selling the item(s) you are dealing with i.e the NPC selling vials.
2. You may need to reopen your profession window(s) with the TSM interface to refresh your recipe cache and populate crafting costs.