O thou, my lovely boy, who in thy power
Dost hold Time's fickle glass, his sickle hour;
Who hast by waning grown, and therein show'st
Thy lovers withering, as thy sweet self grow'st.
If Nature, sovereign mistress over wrack,
As thou goest onwards still will pluck thee back,
She keeps thee to this purpose, that her skill
May time disgrace and wretched minutes kill.
Yet fear her, O thou minion of her pleasure!
She may detain, but not still keep, her treasure:
Her audit (though delay'd) answered must be,
And her quietus is to render thee.
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“[Q]uietus here means ” discharge of payment or settling of a debt .“ This fits with the language of the previous line and the reference to "audit.” 
 The debt repayment is death, of course. In  Hamlet  the Prince speaks of  quietus  with a bare bodkin, that is death by suicide. Here, the payment will be made less willingly to Time or Nature.
Sonnet 126