According to the mum of my girlfriend those times are as follows. You move in together not before 6 months since dating are over. You may pull your girl on my sofa any time after dating her if she wants, but do not marry her before you two lived at least 2 years together and built at least four sets of IKEA furniture as a combined effort. If the relationship doesn't break due to this you can expect a marriage to work for some time. But never promise what is impossible to do: That she will always be your lifelong one and only love. It is a lie. You will love other humans and your relationship will face many ups and downs and love will wear thin and grow again if it survives the thin times. But there is no guarantee for this to happen. But I can guarantee that it will break several times during your relationship with the risk of being over forever at each breaking points. That much is sure.