Q:

What is the longest fork that had occurred so far?

What is the longest fork that had occurred since the start of Frontier?

A:

The longest fork occurred in late August, 2015, a few weeks after the genesis block, on block 116522. The fork was caused by a bug in geth which caused account data to not be properly deleted after a smart-contract SUICIDEed. I'm not sure how long the fork became, but more information about the bug can be found on the blog post

