ころ vs ごろ

[ronsharpe]

In reading Marshdrifter's question about ころ　vs　とき I realized that I was familiar with the expression as　ごろ. This seems to be common in Japanese where voiced and unvoiced consonants are freely interchangeable, giving what I would interpret as two different spellings and two different pronunciations for the same word. Is there any difference in usage between voiced and unvoiced versions of these words? When would I use ころ　vs　ごろ？

[Bueller_007]

In reading Marshdrifter's question about ころ　vs　とき I realized that I was familiar with the expression as　ごろ. This seems to be common in Japanese where voiced and unvoiced consonants are freely interchangeable, giving what I would interpret as two different spellings and two different pronunciations for the same word. Is there any difference in usage between voiced and unvoiced versions of these words? When would I use ころ　vs　ごろ？

As is usual when changing from unvoiced to voiced like this, "goro" is the way that "koro" is pronounced when used as a suffix. "Koro" is a standalone word, "goro" is a suffix.

http://tinyurl.com/yjuyvu
「ごろ」の形で他の語の下に付いて、接尾語的に用いる。
(ア)時を表す語に付いて、その前後を漠然と示す。
「一時―帰る」「二月―できあがる」「一六〇〇年―」
(イ)動詞の連用形に付いて、そうするのにちょうどよい状態である意を表す。
「桜は今が見―だ」「食べ―」
(ウ)名詞に付いて、その面でほどよいの意を表す。
「年―」「値―」「手―」
(エ)年・月・日などの語に付いて、かなり時間の経過したことを表す。
「年―も御祈りなどにつけ、語らひ給ひけれど/源氏（夢浮橋）」「月―隠させ給ひける本意/源氏（夢浮橋）」

So that means that it is used for the following reasons:
1) to represent time approximately (after a time word)
2) to represent the perfect time to do something (after a -masu stem)
3) to represent appropriacy/"goodness" of something's level/degree (after a noun)
4) to represent a long passage of time (after words like "day", "month", "year", etc.) *THIS IS VERY RARELY USED*

If your Japanese is up to the task, you should be using a Japanese-Japanese dictionary for questions like this. If your Japanese isn't up to the task, using a J-J dictionary is a good idea to help it get there.