If you subscribe to the general orthodoxy - no judgement here about whether that is a good or bad thing - a (romantic) relationship lasting tends to be the measure of its success. I can see the appeal and I am not opposed to being - in theory at least - in a relationship that lasts for the rest of my, or their life. I am opposed, however, to the idea that if a relationship ends before then, it's chalked up to failure. Why? My criterion for a successful relationship is one that survives the break-up in that you can, to a greater or lesser extent, be friends afterward.
What are your criteria?