The opening hours field specifies when the amenity is open. Use 24/7 for facilities working non-stop. Examples of more specific intervals:

• Mo-Fr 10:00-21:00
  A single interval is list of weekdates ("-" for a range, "," to add another range), then a space and a time range. A single "Mo-Su" can be omitted, e.g. 11:00-20:00.
• Mo-We,Fr 09:00-13:30,14:00-18:30
  Use commas to add ranges, e.g. when the amenity has a lunch break, or is open on non-consecutive weekdays. Do not insert extra spaces.
• Mo-Fr 09:00-20:00; Sa-Su 10:00-19:00
  A semicolon separates different definitions for different weekdates. Note the "09": you must always use two digits for hours.
• Mo-Su 10:00-19:00; Jun off; Dec 25 off
  You can exclude days or even months with an "off" word.
• Mo-Fr 17:00-03:00, Sa-Su 18:00+
  Time intervals can span midnight, and it is safer to use comma instead of a semicolon in such cases. The plus sign means closing time is unspecified.

More examples, rules, syntax and specifications can be found on a wiki page for the key.