Before I tell you what to do, let me put on table the pros and cons of changing the theme of your blog frequently.

Pros

• It makes the blog look fresh.
• You can stay up-to-date with the recent trend.
• A new theme will add newness to your blog.
• It will break the monotony.

Cons

• A change in the blog theme may weaken your brand.
• Your loyal readers may get alarmed (slight chance).
• Your readers will have to re-acclimatize with the new design.
• It may be perceived as a change in owner or deviation from the proven path.

Now, as we have the pros and cons of change in the blog theme in front of us, we should consider the scenario when the blog theme should be changed.

• When making a big shift, content-wise, or strategy-wise. For example: you have decided to convert your one-author blog to multi-author blog, or you have decided to give it a magazine layout, or you may have decided to add video contents, which was not possible to do without changing the theme.
• If your blog is as old as Uncle Sam. If you have been blogging since the age of Gutenberg then you need to change your blog theme to incorporate the inventions and developments ever since. The new theme will help you connect with the new audience.
• When you want to shift your focus to another group of audience, demographic or psychographic shift.

In any case, you should not change your blog theme many times a year. Change your theme at max once a year. But, in special cases like your blog is not working well with the current theme or for the current group then you can reduce the frequency, but as soon as you find the theme that works well, stick with it.