The length of time it takes to find a winner depends on several variables:
- Number or visits
- Number of conversions
- Number of test versions
- Anticipated improvement in conversion rate
There are two good calculators you can use. Google's calculator will tell you how long it should take to find a winner:
Ophir's calculator will tell you the actual confidence level based on your current numbers. It also provides estimates for future confidence levels if you let the experiment continue running.