ころ・
ごろ after a time is "around" then: 三時ごろ, around three. ころ is a loose "when", most often for a time of life you look back on: 子どものころ, when I was a child.
back when
When I was a child, I often spent summer vacation at my grandmother's house.
Around a time
Time + ごろ
A clock time, a day, a date, a month, a season: 三時ごろ, 十日ごろ, 四月ごろ, 春ごろ. に can come after it or be left out: 三時ごろ（に）.
Back when
Noun + の + ころ／verb or adjective + ころ
学生のころ, 若いころ, 日本に住んでいたころ, 大学生だったころ. ごろ and ころ are also written in kanji: 三時頃, 子どもの頃.
Around a time: ごろ
The results will come out around next Tuesday.
The cherry trees begin blooming around the beginning of April.
The meeting will start around seven.
七時ごろ, 七時間ぐらい
ごろ is for a point in time. For an amount, such as a number of hours, "about" is ぐらい.
I arrived at the station around seven.
A point on the clock.
It took about seven hours to reach the station.
An amount of time.
In conversation you will also hear 七時ぐらい. The reverse, 七時間ごろ, is never said.
Back when: ころ
ころ is "around the time when", with no clear edges: your childhood, your student days, the years you lived somewhere.
When I was young, I used to go mountain climbing every week.
When I was a college student, I worked at the bookstore by the station.
When I lived in Japan, I ran through this park every morning.
ころ or ごろ?
Both are "around then". What comes just before decides which it is.
I'll come and pick you up around six.
Straight after a time: ごろ.
I'll come and pick you up around the time you finish work.
After a verb: ころ. It can look ahead as well as back.
After の or an adjective it is ころ too: 子どものころ, 若いころ.
Words you'll use every day
何時ごろabout what time
About what time will you be home?
いつごろabout when
About when did you come to Japan?
今ごろabout now; by now
They've probably reached the airport by now.
去年の今ごろ is "this time last year".
このごろthese days; lately
It's been raining a lot lately, hasn't it?
A fixed word, with ごろ. あのころ and そのころ have ころ.
あのころback then
Back then, there were no smartphones yet.
Close neighbours
くらい・ぐらいN5
"About" for an amount: how long, how far, how many.
It is about a ten-minute walk to the station.
ときN5
Plain "when". It can point at one exact moment; ころ is always "around then".
Please turn off the light when you go to bed.
Check yourself
1Which one fits?
東京まで
Choose an answer to see why.
2Which one fits?
子ども
Choose an answer to see why.
More detail
見ごろ, 食べごろ
After the stem of a few verbs, ごろ is "the best time for it": 見ごろ (at their best, said of blossoms and autumn leaves), 食べごろ (ripe, ready to eat).
The cherry blossoms in the park are at their best now.
More sentences
The new library is scheduled to be completed around autumn.