くらい or ぐらい (more common in spoken language) has several different uses in Japanese, but is often translated as ‘about’, or ‘approximately’. It comes from the kanji 位（くらい）, which literally means a ‘rank’, or ‘grade’ of something. This use of くらい comes after numbers, or counter words.

• 後（あと）３分（ぷん）くらいで着（つ）く。
  I'll arrive in about 3 minutes.
• ４個（こ）ぐらい欲（ほ）しいな。
  I want about 4.
• どのくらいで着（つ）きますか？
  How long will it take for you to arrive?

As can be seen in the last example, くらい may also be used after question words such as どの, when asking about the extent (grade) of something.
Due to the original meaning of くらい being ‘rank’, or ‘extent’, it may not be used when referring to broad periods of time. ころ will be used in these situations. However, くらい and ころ may be used interchangeably when referring to a specific time (due to a specific time may being able to be thought of as an extent).

• ８時（じ）くらいでもいい？
  Is about 8 okay?
• ８時（じ）ごろでもいい？
  Is about 8 okay?
• 子供（こども）のくらいにディズニーに行（い）った。
  About when I was a kid, I went to Disneyland. (Incorrect example, with unnatural English to match!)
• 子供（こども）のころにディズニーに行（い）った。
  I went to Disney when I was a kid.