Irrespective of whether it is an A/B test, multivariate test or split URL test, you can use our online test duration calculator to estimate number of days to run the test and consequently the amount of traffic you need for a test. Note that the calculator tells you MAXIMUM number of days you should run a test before giving up on it. If within this duration no winning combination is found, it can be easily concluded that there is no combination which beats the control (and hence test can be stopped). Statistically significant results can, however, arrive any time during running the test.
As a thumb rule we suggest running a test at least for 7 days (to account for weekday and weekend effect) before arriving on any results or conclusions.
If you want to calculate the test duration in Excel you may also download the sample excel spreadsheet from this blog post.