How much oxygen does a plant use up at night?
Roughly, half of the CO2 assimilated annually through photosynthesis
  is released back to the atmosphere by plant respiration (Gifford,
  1994; Amthor, 1995).


Source:

https://academic.oup.com/aob/article/94/5/647/151785/Plant-Respiration-and-Elevated-Atmospheric-CO2

Furthermore, it appears like plant respiration rates do not change much with changing temperature, and that larger plants have higher respiration rates.