There is most probably no hard rule that will fit (almost) every hospital, because it depends on a variety of factors. Central hospitals with a lot types of surgery will have a bigger pressure to work on elective surgery. Also, the categories elective versus non-elective can be misleading: On a very strict level elective surgery would include every type of surgery not immediately designed to save a life, while on a more relaxed level it could include only those operations where patients are able to stay at home versus where they would have to stay in hospital waiting on surgery (see also Wikipedia: https://en.wikipedia.org/wiki/Elective_surgery, where there is also a term called 'semi-elective surgery').
With regards to the 'pressure' mentioned above, this could come from a variety of sources as well, such as the community, politics or financial reasons (some surgeries yield a lot more money than others). Of course, health of patients may be a factor: What at the moment may be a purely elective surgery down the road may become an emergency, that not only will have to be operated on, but also diminishes chances of a positive outcome somewhat.
Coming back to a (kind of) ruling when to re-open elective surgeries, most of these will have to be decided on a case-by-case basis, however, for some categories of surgery there is a practically non-existent danger of becoming emergencies, even if quite some time passes, e. g.: arthroscopy. On the other hand any currently elective heart surgery may have the patient deteriorate over time.
For some types of surgery intensive care monitoring is required, which in times of COVID-19 may be of severely limited availability as well.
In closing I believe the re-opening of elective surgery will be gradual and consider more eminently endangered categories of currently elective surgery first.