It will vary. Different ortho's have different ideas. It will also depend on how stable your results are and if you have any bonded retainers.
Retainer wear is for life ... due to our teeth continuing to move for as long as we're alive. Most people start with a few weeks/months of full time wear, then it might be very night for a few more months, followed by continuing that for life, or wearing of your retainers for a few nights a week, like every second day.
Essix retainers are now the choice of many orthodontists, due to higher compliance with a less noticeable appliance.