I can find my deleted questions and answers provided I have enough rep on a site.
Would they always be on the site for me to undelete? Or, does the system purge such posts time to time? If yes, what are the schedules of such purges?
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Content is never hard deleted unless an SE employee explicitly accesses the database and wipes out the data, which is only ever done under exceptional situations in which it's essential that the content not be retained (for example, if there's a DCMA request to remove the content because SE doesn't have the right to host it).
Outside of that, the content stays around forever.