Researchers from the University of Bern in Switzerland found relationship satisfaction hit a low point at 10 years, before climbing to another peak at 20 years.
After this, satisfaction will begin dropping again.
The researchers studied 165,000 adults in 16 countries, asking questions about their relationship satisfaction at different times.
The study's lead author, Dr Janina Buehler, explained the lack of satisfaction that occurred because couples "talk less effectively" throughout the relationship.
"Also, they might show less affection and sexual arousal for each other. Over time this makes partners less satisfied, so many couples will separate," Dr Buehler said.
But the study, with was published in the journal Psychological Bulletin, had good news for those who could stick out the low points, with satisfaction increasing in the second decade.
The team hopes the study will "counteract unrealistic expectations" of a relationship and help people understand that it is normal to experience drop sin satisfaction in a relationship.