Provided it is given the conditions it needs/prefers and not overcrowded, indefinitely. The one in my old garden has been place for about 30 years and is still going strong.

And the current owners cannot be described as 'gardeners' in any sense of the word!!

What if it is overcrowded and neglected?

Well, the one I referred to is certainly neglected!! I don't think they even water. Seldom weed. And no doubt somewhat crowded as well, as it was planted in a smallish space and with a bunch of other rather robustly growing perennials.

Never go back to visit a former garden!! It can be very disheartening. I would avoid it altogether except that I have to pass it directly when visiting friends or running errands in that area.

Thank you for your comments. They are very encouraging to me :-)

The one I am talking about is in my own garden in the bed being gradually taken over by the Blue Spruce tree getting closer and closer to the bed. All plants growing there have to be moved. I was not sure if the Helenium is worth moving or not. I like it but it is over 10 years old.

I've planted supposed species Helenium autumnale and especially named cultivars in numbers of mixed perennial beds, in several cases, maintained by me over periods of around 20 years. With appropriate growing conditions, I find that both the species and cultivars can be long lived, but sometimes the need for periodic division and replanting with soil upgrading (sometimes even every few years) is quite marked. In the latter case, without such attention, the plants get shorter and shorter (year by year) and eventually peter away.

Yes, I have noticed my Helenium is getting less and less vigorous being overcrowded and underwatered and with increasing root competition from the tree. Hopefully it will get better after transplanting.