It's about 4 months until .NET 7 ships ("GA") in November. dotnet/runtime will be following the same procedure that we did for .NET 6, as follows --

• 12-July -- we branched main for Preview 7 and the team has now changed focus from feature development to open bug fixing for .NET 7
• 15-Aug -- we branch main for RC1. After this point, main will be a .NET 8 branch. We will selectively backport fixes to the .NET 7 release branch as needed, with a process similar to the one we use for servicing (but a little more freely). Many or most changes in main will not be backported. No features will be backported.
• 13-Sept -- we branch our release branch for RC2. Nothing changes for those working in main. The bar for changes to be ported into .NET 7 becomes considerably higher (centralized permission process of taking to .NET tactics meeting)
• 10-Oct -- last ship stopper fix merged if any.

Where do community changes go? main remains open at all times for community changes, even adding new approved API. Many will get into .NET 7 if they go in before 15-Aug. However if a change brings too much risk we will hold merging it until 16-Aug.

TLDR: if you plan to work on a change that you want in the .NET 7 product, 15-Aug is the date it needs to be merged by.

Nearer the fall, we'll start thinking and planning for .NET 8, and we'll be sharing our thinking when we do.