But did you know that Google now treats 429 error codes the same as they would treat 503s?
A 429 code means "Too Many Requests (RFC 6585)," "the user has sent too many requests in a given amount of time. Intended for use with rate limiting schemes."
John Mueller from Google said this on Google+, "it used the be that we'd only see 503 as a temporary issue, but nowadays we treat them both about the same." Both meaning the 429 and 503.
John said "should I use 429 or 503 when the server is overloaded?" Either is fine, "we see both as a temporary issue, and tend to slow down crawling if we see a bunch of them," John added.
John added that if the issue is longer than temporary, then Google may drop off the URLs from the index but they will try again shortly after.
If they persist for longer and don't look like temporary problems anymore, we tend to start dropping those URLs from our index (until we can recrawl them normally again).
Forum discussion at Google+.