冰冻三尺，非一日之寒
bīng​dòng​sān​chǐ​fēi​yī​rì​zhī​hán

Rome was not built in a day

冰冻三尺，非一日之寒 冰凍三尺，非一日之寒 bīng​dòng​sān​chǐ​fēi​yī​rì​zhī​hán
Rome was not built in a day
Literal meaning: ice three feet thick is not formed by one day of cold; used to express that a current situation (often a problematic one) is the result of long-term accumulation and will not be easily resolved.