Syncthing follows the Semantic Versioning scheme of versioning. Each release has a three part version number: major.minor.patch.
A new major version is released when there are incompatible API or protocol changes, a new minor version is released when there are new features but compatibility with older releases is retained, and a new patch version is released when there are bug fixes (compatibility with older releases always retained).
While still in pre-release mode, i.e. versions 0.x, breaking changes are made in minor releases rather than major. Version 0.7.3 should be able to talk to version 0.7.52, but will probably not understand version 0.8.0. This also means that if you don’t like 0.7.52, you can safely downgrade to 0.7.3 again and keep your configuration, index caches, etc. However 0.8.0 might have a different format for those things so a downgrade to 0.7.x might be trickier.
A new patch release is made each Sunday, if there have been changes committed since the last release. Serious bugs, such as would crash the client or corrupt data, cause an immediate (out of schedule) patch release.
Minor releases are made when new functionality is ready for release. This happens approximately once every few weeks, with the pace slowing as the 1.0 release nears.
A new major release is a rare event. At the time of writing this has not yet happened and is foreseen to happen only once in the foreseeable future - the 1.0 release.