Hi!
A great question!
If you are focusing on long-term performance, then 3 months is the right choice, as this is enough time to consider whether your Keywords and overall content, and SEO strategy is making any progress, or needs improvement.
For shorter spans of time such as holidays, and other important time-bound events, of course, more frequent evaluations are advisable, as you can quickly fix any gaps in your marketing approaches.
Regards,
ZETTAHOST