But what is 'True Love'? Each of us may consider somehing else...
If you talk about those sweet feelings, that tingling in stomach and beating heart, they may last some time. But eventually they will fade away... How long does it last, 2 months? 3 years? It depends, but eventually those feelings will be gone...
What is left then? Then you have to look at the overall relationship. Who contributes what, how much effort both invest, what are their common things to do... There are many factors, for example religion may at first not be important at all, but after 3-4 years it may become a huge issue... The girl may be at first very attractive, but after say 10 years in relationship she gained 30-40 extra pounds and she gave birth to 2 children... She used to pay 100% attention to him only, but now she pays 80% attention to children and only 20% to him... She is no longer visually attractive, yet there are still plenty of young females around... If she had to chose between children and you, guess who would she chose...
So when you take the initial relationship and compare it to 10 years later, there are huge differences... During the 10 years, we meet new people, we get different jobs, we learn new things that are now important... We may grow easily apart because life is constantly changing...
The rule in seduction usually is NOT to fall in Love first (if at all), because if you do you will usually mess up and lose... You will start overinvesting, carrying too much, chasing and so on, and that will push her away to auto-rejection... The rule is, the person who falls in love first usually loses...
So if you are looking for some pattern, IMO the best is to watch how much are both investing into each other, emotionally, mentally, physically... Which requires constant and persistant effort... Ideally you want to be investing just little bit less than she does, that way you always have upper hand...
Initially you will (ideally) investing some 30% overall, while she 70%. As the time go, you can let it go down say you 45% vs her 55%... Said in other words, if you want to keep her she always has to be investing into you...