gurai is used after a noun: nan JIKAN gurai kakarimasuka. (How many hours will it take?) here, jikan is a noun and it means "hour" nan kurai kakarimasuka. (how long will it take?) here, nan is an interjection, so we use kurai same for koro and goro and yes (please, don't let this question available for further answering, if you find your best answer, choose it as so. ^^)

about "ha" and "wa", what you said it right ( eventho they are slightly pronounced differently in a decent way.) and "soredeha" ha is a particle. that's why it's wrote as "ha" in it.

“- Kurai, - gurai” are all vague expressions used quite frequently in Japanese. They mean “about” or “approximately” and such indefinites are expected to be used even in very definite situations. For example, even if you have spent exactly fifty “hon” (本), one would still say “Hon ga gojussatsu gurai arimasu” (本が五十冊ぐらいあります。) – “There are approximately 50 books” . Their use is practically required in answering a question which has been asked with vagueness, such as “dono gurai” (“how long”, “how much”, “how many”). And one good link for teaching pets ;) - http://www.maggiesensei.com/2011/01/26/request-lesson-%E4%BD%8D-%E3%81%8F%E3%82%89%E3%81%84%EF%BC%8F%E3%81%90%E3%82%89%E3%81%84-kuraigurai/

You want to know when to use which one, kurai or gurai, right? I think it is influenced by the proceeding sound, but in most cases, both can be used.