Regardless of how long your trial runs, people really need to bake in resource limits. I assume most SaaS companies have offered (or will offer) tiered rate plans based partially on consumption, so if you are starting as a free-only beta, why not bake in some sane resource limits from the start? Some ideas for this might be:
* maximum podcast size/duration (10 min or 3MB free?)
* 4 free uploads, then you pay
* Retention period - free accounts only store data for 30 days from upload
Sure, you are still technically paying for the free folks, but you greatly limit the cost damage while providing reasonable motivation for people to move to a paid tier when they are ready.