The duration of a heatwave varies significantly by region and specific weather patterns, but typically heatwaves last from a few days up to a couple of weeks. For example, historical heatwaves in the U.S. have ranged from 3 days (1995 Illinois heatwave) to 17 days (1938 heatwave), with some extreme cases lasting a month or two

. Regarding current forecasts for late May to early June 2025:

• In parts of India, heatwave conditions are expected mainly during the week of May 22-29, 2025, with a high probability of heatwaves in some regions early in this period but a NIL probability of heatwaves during the following week (May 30 to June 5)

• In Europe, early summer 2025 is predicted to have above-normal heatwave potential, especially mid to late summer, but the spring-to-early summer transition period will be dynamic with some variability

• For the U.S., extreme heat is forecast to persist through July into early August 2024, with no significant cool-down in sight during that time frame

. (Note: This is for 2024, but it illustrates that heatwaves can last several weeks.)

• In the UK, a mini heatwave in early May 2025 lasted about a week before temperatures returned to normal

In summary, current forecasts suggest that heatwaves in various regions during late May 2025 may last about a week or slightly longer, with some areas expecting heatwave conditions primarily in the last week of May but not extending into early June

. The exact duration depends on local weather patterns and can vary from a few days to a couple of weeks or more in extreme cases