A sentence of 13-30 years means the sentence cannot be less than 13 years or more than 30 years. In Florida, there is a point system used to determine whether a person is eligible for the minimum. Casey would not be eligible for the minimum because of her prior criminal record (the six felonies). Each prior conviction, as well as each new conviction, adds points and the higher the points go the higher the sentence must go...up to the maximum of 30 years.
Casey will likely get credit for time served so if she were to get a sentence of 30 years, she has already served three years of that. (There is a possibility of sentencing in this current matter being consecutive to what she is currently serving time for, in which case there would be no credit for time served.)
If Casey is convicted on more than one of her current charges she will probably get concurrent sentences. If, for example, she gets a manslaughter conviction and receives the maximum of 30 years to run concurrent with sentences for all other convictions, she might serve a little over 25 years in prison (after the 85% rule is applied). From that 25 years she might get to deduct the three years she has already served...so she could be out in a little over 22 years.
Should Casey be convicted on the most serious charge she gets either LWOP or DP. With either sentence, she is likely to never be free again. I really do not see the DP happening in this case, unfortunately, so unless Casey gets a Life Sentence, she will get out some day, and whenever that happens will be too soon, IMO.