Usually I have my time mage function similarly to a healer at first who can reverse time to heal wounds. In fact, the character often thinks of his/herself as a healer and discovers her other abilities later (think of Otihime's healing from Bleach). I usually make it so that I can only speed/slow/reverse time in a small sphere close to my body. The further extended the sphere is from my body, the more energy it takes. The larger the sphere, the more energy. The faster/slower the time difference is, more energy. Controlling multiple spheres is near impossible as it takes exponentially increased amounts of energy to perform.
Think of it as a power that starts off extremely weak but, given enough time and training, can be immensely strong. However, Ive never gotten far enough in an RP for the power to develop it past initial stages. :/
Also, taking time off of one's own life might not be drastic enough because, effectively, there isnt a kick back at all.
Even if I cant be a time mage, Im still interested. :)