What do you mean back pay. Baka last pay (unpaid work on the cut-off before you were separated, pro rated 13th month pay and tax return).
Even if you are terminated, you still need to process your clearance before receiving your last pay otherwise, they will not give it to you until you do.
If papadala mo lang ang lahat, ano kasiguraduhan mo na receive nila yung pinadala mo? this can be denied, so the best thing is go to your company to process your clearance.
pwede ipakuha ang last pay mo as long as you comply with their requirements. Some company only require authorization letter, some company require SPA.
and yes pwede umabot ng 1 year and more ang release, ang tanong lang diyan kailan ka natapos magpa clearance.