try and not make it too long!! the VO don't really have time to read long essays. If there is something out of the ordinary in your relationship that needs explaining, then elaborate a little bit. If you have a fairly "straightforward" application, then keep to the main "events" (met, introduced to family, moved in together, got married, kids, etc. - and not necessarily in that order!), to give a genral idea of how your relationship developed into what it is now. Other than that - no need to write anything long!
Sweden