There will be a 14-day period between the Bellatrix upgrade and the mainnet Merge. That’s the length of time that will pass before the network reaches a specific total terminal difficulty (TTD) which, in turn, will activate the Merge. The tentative TTD mark is 58,750,000,000,000,000,000,000. Nothing is final, however, until it’s codified in a client release.
Source: https://www.coindesk.com/tech/2022/08/11/ethereums-merge-now-has-tentative-dates/?utm_medium=referral&utm_source=rss&utm_campaign=headlines