Your trial ends once 12 months have elapsed or you've spent the $300 in credit. The amount of credit and days remaining are displayed at the top of the Google Cloud Platform Console. If you want to extend this 12-months period, or stop incurring charges when inactive as a programmer, you may chose Always Free, if your student-specific usage pattern does not require more than the limits specific to this option, namely:
28 instance hours per day
5 GB Cloud Storage
1 GB of egress per day
Shared memcache
1000 search operations per day, 10 MB search indexing
100 emails per day
More detail on the Always Free limits can be gathered from the "Always Free Usage Limits" documentation
page.
To be admitted to Always Free, you need to fulfill the conditions for admission:
You do not have a custom contract or rate card with Google.
Have an upgraded billing account
Account must be in good standing
An
upgraded billing account is a billing account that will receive automatic charges for any usage beyond any credit on the account. Upgrading your account means converting your free trial billing account to a paid account.
Related information can be read on the "Frequently Asked Questions"
page, relating to the free tier of the Google Cloud Platform. The free tier is available only for the Standard Environment and for certain regions, namely: us-east1, us-west1, and us-central1.