Once you are convicted of a crime and sent to prison the amount of time you are given to spend in prison can still vary depending on different things. If you do nothing and just remain in prison, the sentence will likely not change. You will need to actively pursue a reduction in sentence so that you might be able to get reduced earlier than might be planned.
One way to pursue early release is to file paperwork to ask for a reduction in sentence based off of various problems with the case or different circumstances of the case. Later on, you may also request parole as a way of obtaining an early release from federal prison. Whether you get out of prison or not with parole will depend on your behavior inside of prison.