Insurance is offered each time the dealer has an Ace as his upcard. If the dealer does not have a 10 in the hole, you simply lose your insurance side bet and play continues. Mathematically, it is simply a bad bet both in single deck and multiple deck games. The chances the dealer has that 10 in the hole are almost always greater than 2 to 1. Consider a single deck game, on the very first hand.
Insurance is offered.
At this point there are 14 tens and 33 non-tens left in the deck. Insurance should not be taken. This may help clear it up. Suppose you are keeping track of the cards that remain in the deck. At some point insurance is offered and you realize no tens remain in the deck.