三年之痒七年之痛
[sān nián zhī yăng qī nián zhī tòng]
It refers to an emotional period between couples or people closely connected. The 'itchiness/scratch' represents the difficulties of getting through after 3 years; and if they survive these problems, then 7 years is another hard time, usually indicating a longer-term difficulty with the relationship that might lead to breakups.