Most chemical burns last between 3 to 7 days before they go away.
However it also depends on how bad the chemical burn is and if it's a more severe chemical burn then the chemical burn may take a month or 2 months to fully heal.
But for small chemical burns they usually heal up within a week at most.
The best thing you can do for a chemical burn is to wash the chemicals off the skin when you come into contact with the chemical that caused the chemical burn.
Wash the area with some soap and cool water for at least 10 minutes to get rid of the chemical on your skin and to cool the area.
Remove any clothing or jewelry that has been contaminated by the chemical and wash those or throw them away if needed.
Then bandage the chemical burn or allow it to breath.
If the chemical burn is really severe then seek medical treatment or if the chemical burn lasts longer than a month or seems to get worse.