Over 50% is a simple answer, but if you look back at 2017 BCH, Despite having some major miners, exchanges, and even certain prominent early Bitcoin advocates support it, it still failed to become the dominant chain because it couldn't convince enough of the full node network to switch.

From the 2017 SegWit2x attempt we saw that even ~90% miner signaling wasn't enough when a strong majority of nodes opposed the change. It proves that raw hash power percentage isn't the critical factor.

The miners are incentivized to go with the majority. If a large-scale miner is mining a fork that nobody uses, they're gonna go under pretty fucking fast. Especially if they're trying to push a change and their blocks keep getting rejected by the network as invalid.