This is one of a series of questions dealing with the magic explained below:
In my fantasy world, magic is an energy similar to radiation. It alters the DNA of living cells, causing them to mutate or die. There are people in this world capable of controlling the magic. They can control the change, using magic either to kill cells, or change them in a host of beneficial ways. For example, they might give themselves night vision by altering their own eye cells.
This particular question deals with the speed of the magic. How fast could a magic-wielder work change in his victim? Is it instant, a long, slow process, or somewhere in between?
Remember that magic is for all purposes controllable radiation. It works on a cellular basis and can only affect a cell's DNA. Magic should affect DNA at the same speed radiation does. Disregard for the moment any time required for the magic-user to control the magic.
Please let me know if additional details are required.