|MadSci Network: Botany
It is impossible to say without a specific measurement. Plants generate oxygen (O2) during photosynthesis according to the equation 6CO2 + 6H2O ===> C6H12O6 + 6CO2. Any factor that affects photosynthesis will change the rate of oxygen release. Some of these factors include amount of light and water available, soil nutrients, temperature and even specific characteristics of the plant.
However, it is possible to estimate the amount of oxygen released by the plant with some basic mathematics and simple assumptions. If we consider that 80% of the fresh weight of the plant is water and aproximatelly 40% of the rest is carbon, for every 150 grams of plant tissue grown, 32 grams of oxygen are released. If you know the weight of your plant, you can calculate how much oxygen it released in the atmosphere during its whole life. Just to give an idea of how much is that, 32g of oxigen means 22 liters of oxygen under the usual conditions of temperature and pressure (try to imagine how many bottles you can fill with that!).
Try to observe the growth rate of your plants (how fast they grow) and you will see that each plant have it's own speed of growth. Don't forget to count the roots. With some observation you will see that the release of oxygen is acctualy very slow and that it is very dificult to talk about an "average plant", since each plant has it's own characteristics and physiology.
Try the links in the MadSci Library for more information on Botany.