Does anyone know how long 'auto-tune' actually runs the test? I'm hesitant to select it as there isn't a set time to run the test before auto-tune takes over and starts displaying the winning campaign.
From the Product Docs: Selecting Auto-Tune allows the RTP platform to automatically recognize the better performing split test campaign and continues with the highest converting campaign, while pausing the others.
I suspect in this case that you need to setup a reminder to look at the data after however many days.
This will depend on the site traffic to achieve whatever confidence level Marketo has set as default (prob 95%).
AB Test Your RTP Campaign - Marketo Docs - Product Docs
I recall some other products with auto-tune will start to display the most likely winner within a couple of days if there is enough data, for example, LinkedIn AB tests do this.
RTP will run the test until it has a statistically significant difference. Once it reaches that point it will recommend the winner of the test. The duration is determined by the results it receives