Some flower occasionally, most have never flowered. That being said, I've only had the majority of them for less than five years. One flowered for the first time after almost 10 years. It hadn't even occured to me that it could!
Hosta sieboldii has flowered most often, about 3 times I think. It doesn't grace me with flowers every year though, and it was at least 5 years old when it flowered for the first time. To me Hostas are foliage plants and flowers are just a very rare bonus ;-)