They can fork at any time, but that would make it pointless.
The hard fork will be triggered only when they have substantially over 50% of the hash rate. (Although some miners may be tempted to trigger it as soon as they cross 50%). What the hash rate really tells us is the support they have amongst the miners for increasing block size. This should be a wake up call for Bitcoin Core as well.
I dont think bitcoin unlimited is supported by all the bitcoins miners. Its only the few miners who would like to increase the block size on the expense of bitcoin core. Lets see where it turns up now.
Wouldn't really go ahead and say that they're only the "few miners who would like to increase the block size" as they do have a pretty high amount of miners on their side, I think its in the realm of 34-37 percent of miners backing them when they only need to get up to 51 percent in order to initate a hard fork of the chain.
I highly doubt this would happen though, as they'd want to make sure that nothing could stop them and they'd probably want around the mark of 70 percent so even if some miners leave they still have a good amount of control.
They could fork at anytime, though it wouldn't really do anything if they're below 51 percent as they wouldn't have a hash majority.