My Mini Daschund came in heat at six months, and has not come in again. She is fifteen and a half months old. Is this common?

Twenty one days is the normal cycle no matter what kind of dog.

Tick is referring to the actual time in estrous, not the time between estrus periods. (I think)

In your case, if you have not missed one of her estrous periods, it is not normal. Anything over 15-20 months between cycles is abnormal. But be prepared to jump through numerous diagnostic hoops in hopes of finding out why. It may be a progesterone secreting ovarian cyst, a malfunctioning ovary, other hormonal problems besides ovarian, such as thyroid. Or other medical problems.

What if you leave it alone? It may well depend on why she is as she appears to be. (Still haven't decided on whether you may have missed her recent cycles) Maybe nothing. Maybe puppies if you decide she won't come into heat, but she decides to... at the wrong time? If low thyroid, there are other things that problem brings up, and if corrected she may start normal cycles.

So, if you know you want to TRY and make money with mini-daschund pups (consider how far in the hole you will have to start, if you do) go for the testing and good luck!

Otherwise, just have her spayed and watch her diet so she doesn't get too heavy. Also, include a thyroid test somewhere in her future, like pre-op?