The amount of time it will take you to reach 99 magic, depends entirely on how much time you spend playing RS each day. If you are dedicated to it though and spend a solid chunk of your day each day training, then it shouldn't take you any longer than 2 weeks. It is very possible to do it in far less than 2 weeks aswell
Just a tip - turn on your computer's mouskeys feature, and use the numpad number "5" as your mouse. Then set up your computer up near a TV and just watch movies while you softly press the "5" key repeatedly. Much easier than clicking, and much less strain on your wrist and fingers. This allows you to alch without paying any attention for hours at a time.