Text-to-speech free tiers don't meter minutes of audio — they meter input characters, and the conversion isn't intuitive. English averages roughly 1,000 characters per minute of speech, so a "1M characters/month" tier is ~16 hours of audio; a "10,000 characters" trial is ten minutes.
This is why provider rankings flip depending on your content. Long-form narration burns characters linearly and wants the biggest character pool. Short interactive prompts (an agent speaking one sentence at a time) hit per-request overhead and rate limits long before the character pool — a smaller allowance with higher RPM wins there.
Also worth checking per provider: whether SSML markup counts against the quota (tags are characters too) and whether re-generating the same text is cached or re-billed.
The current tiers converted into actual audio-hours: https://toolfreebie.com/free-text-to-speech-api/
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