You may feel some improvement within a few months, though actual muscular change happens more slowly. A recent study looked at patients who did one of three exercise therapies (physical therapy, training with special devices, or aerobics) twice a week. After two months, these patients did better on tests of physical strength and endurance. However, the improvements didn't necessarily show up at the muscular level. The size and type of muscle cells inside the muscle hardly changed at all in three months' time. The improvements most likely had to do with better muscle action and coordination.
Researchers believe that it may take more than a few months for muscles to change, especially if you haven't been active for a while. But in the meantime, you'll get the benefits of better strength and endurance. Plus, you'll be on your way to a more successful and complete recovery.