The time it takes to temper.

Tempering chocolate is similar to riding a bicycle; once you learn how you never forget.  Even the most experienced chocolatiers make mistakes, especially when it is close to a meal and for reasons unclear, the chocolate is not tempering properly and you have been waiting for an hour for it to work its magic. So everyone walks away and returns thirty minutes later to find the chocolate is now overtempered, a thickened, hardened mass that defies gravity, climbing up and out of the machine like the blob. The clean up is terrible.  The lesson well learned; never walk away from the task at hand.  And as important, you have learned the other lesson; get back on the bike and start again, because you still have chocolate to temper and candy to create.