decade

in the 50's

the 1970s (the '70s)

not come across a leap year in the last decade

once in a blue moon; chance of a lifetime; very rare occurrence

in the 30's

for the past thirty years

(the basic law in HK) fifty years without change

in the 90's

during the 90s (era)

Listening to what you said is better than studying for 10 years

in the 80's

during the 80s (era)

several decades

several decades [written and colloquial]

loss of skills if not used for a long time

once in a blue moon [colloquial]

(literally: a fortune teller may deceive you for only eight or ten years); time will tell who you can trust; you will realise with time that I am telling you the truth (whilst others may lie to you); time will witness the truth; would I lie to you? [colloquial]

it takes ten years to grow a tree (...but a hundred for a person)

lit. ten years of the tree wood, meaning it takes ten years to grow a mature tree

in the 60's

in the 1940's

during the 40s (era)

in the 1920's

(literally: a Fengshui master may deceive you for only eight or ten years); time will tell; you will realise with time that I am telling you the truth; time will witness the truth; would I lie to you?

people and things change a lot in the space of ten years [written and colloquial]

literally: 'a Fung Shui master can fool you for only eight to ten years, but not for a whole life'; time will tell; you will realise with time that I am telling you the truth; time will witness the truth; would I lie to you?