TDEE and exercise

Sorry if this is a silly question. What classes as "exercise" when working out your TDEE. I have put I do "light exercise" as I do 30 mins of cardio 3 times a week - is this enough or do you have to work out for longer? I can't see anywhere how long classes as exercise or is it just a question of something that gets your heart going so many times a week?
Thanks all.

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