There isn't a hard and fast rule regarding the exact character limit for image alt text (also known as 'alt tags' or 'alt descriptions') in SEO. However, best practices suggest that keeping your alt text around 125 characters is a good rule of thumb. This is due to some screen reading tools cutting off alt text at around this length.
The primary purpose of alt text is to describe images to those who cannot see them. This might be because the user is visually impaired and using a screen reader, or it could be due to a slow internet connection or an error that causes the image not to load.
When writing alt text, the focus should be on providing clear, concise, and useful context for the image rather than worrying too much about character count.
Here's an example of how to add alt text to an image in HTML:
While there's no penalty from search engines for longer alt text, remember that the main goal is usability and clarity.