It will last for five turns if created using Drought (Generation VI onwards) or Sunny Day, or eight if created by a Pokémon holding a Heat Rock. If created using Drought (prior to Generation VI) or encountered in the overworld, it will last until replaced.
There is also a variation of harsh sunlight called extremely harsh sunlight (Japanese: 日差しがとても強いvery intense sunlight). Extremely harsh sunlight can be encountered in the overworld or created using Desolate Land; if the latter, it lasts until the summoner is removed from the field (and no other Pokémon with Desolate Land remain on the field), the summoner is rotated out in a Rotation Battle, the summoner has Desolate Land suppressed or replaced, or heavy rain or a mysterious air current are created by Primordial Sea or Delta Stream respectively.
The screen brightens and dims alternately. The screen may also turn a shade of orange that flows while dark spots ride across.
Extremely harsh sunlight
The horizon turns orange and yellow. The screen turns to bright, daytime appearance, regardless of actual time, with the upper portion of it becoming especially bright. Pillars of light randomly appear.