If you are asking what the length of the blockchain is now, it is currently (6440717)[https://etherscan.io]6440717 blocks high at the time of writing.
If you are asking what the longest possible length of a forked chain can be, the answer is infinite. Forked chains will continue for as long as there is at least one miner mining on that chain.
Take ETC for example. The block height of ETC (a fork of ETH) is 6,667,943 at the time of writing. ETC forked from ETH on block 1,920,001, meaning the current height of this fork (further than the ETH height at time of fork) is 4,747,942. This can go on for as long as miners will mine ETC.