Thanks for reaching out to us. There are two kind of free trail, one is free trail account, one is free tier pricing, I think you are mentioning free tier pricing of the service.
Actually for Speech Service, there is no time period for the free tier, but for the training limitation, as below table you can see, for speech to text custom model, there is a limitation for audio hour and model number. You can enjoy the free training until it exceed the limitation, if you want to do more, you need to upgrade your account.
If you are asking about the free account - We’ll notify you so you can decide if you want to move to pay-as-you-go pricing after your free period, enabling you to purchase Azure services beyond free amounts. If you do, you’ll keep getting access to all the free services. If you don’t, your account and services will be disabled. To resume usage, you'll need to move to pay as you go.
I hope above helps.
-Please kindly accept the answer if you feel helpful to help the community, thanks a lot.