Hi there, Matt! Once a starter reaches it's peak (doubled in size and very bubbly throughout) and starts to deflate it is still usable for leavening bread but the resulting loaf won't be as lofty as the yeast cells aren't as strong at this point. But, once the starter has collapsed back to its normal volume or is just about there it won't be very good for leavening bread. We hope this helps! 

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