will Allah (SWT) destroy us completely after a period of time in jannah or hell? If yes, does that mean we will be nothing?
There will be human's eternal life. Allah (SWT) has not created us to destroy us. It's in obvious contradiction with Allah's wisdom and justice.
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In Islam, Heaven is understood to be a place of everlasting life.
"But those who have faith and work righteousness, they are companions of the garden. Therein shall they abide forever" (2:82).