When will a reusable single-stage-to-orbit vehicle deliver a payload to orbit?

In an effort to drive launch prices down, SpaceX and other "new space" launch providers have been developing and flying multi-stage rockets, aiming towards full reusability of the all stages.

Another concept for reusable launchers are single-stage-to-orbit (SSTO) vehicles. Such vehicles would provide launch cost saving by eliminating the need for different stages, be they reusable or not. However, SSTOs must try to squeeze out the necessary engine performance, while drastically reducing structural weight, and still be able to reliably deliver a meaningful payload in orbit. Several reusable SSTO concepts have been proposed.