The effects of special relativity, including time dilation, primarily apply to inertial frames of reference. An inertial frame is a reference frame in which an object at rest or in uniform motion will continue to do so in the absence of external forces.
When you accelerate to a constant velocity of 99% of the speed of light (c), you would indeed experience time dilation while maintaining that speed in an inertial frame of reference. This time dilation would be relative to an observer in a different inertial frame of reference, such as an observer at rest relative to Earth.
However, it's important to note that the process of acceleration itself introduces additional complexities. Special relativity is not directly applicable to non-inertial frames, and a proper treatment of acceleration requires the use of general relativity, which incorporates gravity and acceleration.
When you accelerate to reach your constant velocity of 99% c, there will be a period of time during which you are changing your velocity. During this acceleration phase, time dilation effects may not be well-defined or may require more sophisticated mathematical frameworks, such as those provided by general relativity.
Once you have reached your constant velocity and are in a state of uniform motion, time dilation will apply in relation to observers in different inertial frames. As long as you maintain that constant velocity, you will continue to experience time dilation relative to observers in other frames of reference.
In summary, time dilation effects in special relativity primarily apply to inertial frames of reference. While maintaining a constant velocity, you would experience time dilation relative to observers in different inertial frames. However, the specific details of acceleration and its effects on time dilation require the use of general relativity.