The best way I could answer this is to describe the way in which I am working at the moment:
I am preparing Ibert flute concerto, Franck violin sonata, an orchestral piano part for a piece by Matthias Pintscher, a Bach organ transcription, a Scriabin étude, and the Schubert fantasy for violin and piano.
It is not rigid, but since I allow myself no more than three hours a day of practice presently, it works out that I spend somewhere around half an hour in each of those areas, though the time needed for each piece diminishes as I progress through the learning process (there are many more pieces and parts that I will have to learn throughout the summer, so some rep will eventually go into a two- or three-day rotation).
With the Ibert, I began by reading through the first movement for a day or two, getting a sense of structure, some fingerings, phrasing, and so forth; then, on the third day I began to practice it in sections, and began reading the second movement.
Basically, I work on several things simultaneously, but in sections, and at a pace in accordance with the "due date." Rarely do I learn more than two or three things at once from scratch, except during the academic year, when I occasionally have no choice (as a Collaborative Piano student).
Hope this is useful to you . . .
Mike