You need about 1000 transactions per month before it makes sense to invest in AB testing. A transaction can be anything you want to improve: a purchase, a page view, a download, a click, a new lead, a phone call, whatever you want to improve.
For example, if you want to improve sign-ups to your newsletter then you must have 1000 sign ups a month before you invest in testing variations. Any less than this and things take too long to test. No one wants to wait for 35 weeks to find out if a change had any impact.
1000 transactions is just a rule of thumb. The actual number depends on your current conversion rate, the size of the change you are trying to detect and how much traffic you get. If you have these variables then this 2 minutes video will show you how to calculate exactly much traffic you need for your site:
1000 transactions per month is a good rule of thumb. If you have less than 1000 transactions your time and attention are better invested in growth without the reassurance of AB testing.