Latest time to reverse a declined PhD offer Got two PhD offers from universities A and B (in US) and chose B. Because I'm working towards my master's at B, I soon got involved in some research work here. However, the experience turned to be disappointing. I'm now regretting my choice and thinking of contacting school A to see if there's still chance to get that offer back. But:

1) The normal deadline to make the decision (on student's end) is April 15. Now it's already summer, how much chance is still there? I assume that A has most likely finished everything about admission.

2) My current professors at B are well-known in the field and have been very nice to me. After I decided they've also gotten me involved in a number of future plans. Moreover, I've informed a bunch of relevant professors (even including those at A) who asked of my final decision. Would changing my mind this late heavily ruin my credit and academic career?

I feel like at this late stage, simply asking would mean I'm determined for a change, so I would not simply send an inquiry email without figuring out the potential risks...
It is likely that there is no longer a slot open for you. But under the circumstances you state, the latest time to contact them is probably now. Once you declined an offer they moved on and accepted others. If those others have been accepted there is no longer room until the next class. 

But if you are willing to wait a year, then you can probably enter the pool again and be considered like anyone else. 

However, it also just sounds like a case of "buyer's remorse". Your situation at B sounds like it would work out fine.