what is the statue of limitation for a 15 year old case?
Different crimes have different limitations periods. Further, under some circumstances, the limitations clock can stop ticking for an extended period of time. You haven't told us what you're charged with or what has happened during the past 15 years. There is no way to offer you meaningful guidance without that information.
You don't tell us about the underlying charges. If it is murder, there is no statute of limitations.
See my other answer to your question re: statute of limitations and whether or not the case can proceed.