New libhandy stable versions should be released for each new stable GNOME version. New development versions should be released roughly montly. Once we are happy with the feature set we have to offer, a 1.0 stable version will be released instead of a 0.x one.
The following roadmap is there to give you an idea of what we want to achieve, we may not update it nor may we follow it closely.
More or less new versions could be released depending on how we think it is relevant, for example a new stable release may acompany the release of the Librem 5. The first stable release of libhandy (acompanying the GNOME 3.32 release) should offer both GTK+ 3 and GTK+ 4 variants.
Several of libhandy's classes will be submitted to GTK+ 4 directly at some underterminded point in the future, but likely not before the release of the Librem 5 phone.