According to Dr Thomas Stuttaford in The Times, there are 3 stages to a successful long term relationship which can be scientifically measured by functional MRI brain scans:
Lust. Lasts between 1 to 2 years (remember we are talking about successful relationships, clearly lust can dissipate after one date and many people have suffered that social worker term S.I.R. – shag I regret).
Acceptance. Lasts up to the eighth year. Couples acquire shared interests and goals and nest together.
Firm friendship. Starts from the ninth year and lasts a lifetime. Or not…