There are actually three different things that are applicable here. There are facts, there are scientific laws that describe the way facts act, and there are theories that explain why facts act as they do.
A scientific law is a concise verbal or mathematical statement of a relation that expresses a narrow fundamental principle of science. A scientific law must always apply under the same conditions and cannot be rigorously applied outside of those conditions.
For example, gravity is a fact that we experience every day. Newton's law of universal gravitation states that the gravitational force between two objects equals the gravitational constant times the product of the masses divided by the distance between them squared.
As used in science, a theory is an explanation or model based on observation, experimentation, and reasoning that describes how certain facts relate to each other, especially one that has been tested and confirmed as a general principle that explains natural phenomena and is capable of predicting additional phenomena that derive from those facts.
That is illustrated by Newton's law of gravity, which merely states how gravity acts, and Einstein's theory of relativity, which describes what gravity is and why it acts as it does. Einstein's theory of relativity, in fact, was verified by predicting a discrepancy in Newton's law of gravity in calculating changes in Mercury's orbit around the sun because of relativistic effects. The discrepancy in Newton's law, though small, was predicted by Einstein's theory as a result of Mercury's elongated orbit and closeness to the sun and the warp in space-time caused by the sun's great mass, something that Newton's law does not account for. This shows that Newton's law breaks down in relativistic settings.
Evolution is a valid scientific theory and has been verified numerous times and never disproved, despite the numerous false claims of creationist otherwise. It has also made numerous successful predictions.
So in answer to your question, the theory of evolution will never become a law because scientific theories and laws are two different things.
Also:
Theory, Sense 1: A scheme or system of ideas or statements held as an explanation or account of a group of facts or phenomena; a hypothesis that has been confirmed or established by obversation or experiment, and is propounded or accepted as accounting for the known facts; a statement of what are held to be the general laws, principles, or causes of something known or observed.
Theory, Sense 2: A hypothesis proposed as an explanation; hence, a mere hypothesis, speculation, conjecture; an idea or set of ideas about something; an individial view or notion.
Evolution is a theory in Sense 1, as are general and special relativity, quantum electrodynamics, quantum chromodynamics, thermodynamics, and so on. The reason some people substitute Sense 2 in a perjorative manner is that they think evolution (but not the other theories) contradicts a sacred text. In fact, it contradicts only a specific type of interpretation of that text.
