We are going to use Kubernetes Engine, Cloud SQL, Storage, and possibly Nosql and cache services for our academic project.
So far we'll have 5 accounts and the project has to be running for 3 months.
Will it be enough for us? Is it possible, for example, to stop project and billing every night or even stop and start at any time?
The Google Cloud Platform (GCP) Free Tier includes $300.00 USD in credits to get started with any GCP product. Since this is an academic project, I recommend using the Google Cloud Platform Pricing Calculator to learn how to price GCP products and services - link. Also review what is included in the "Always Free Products" - link. Then there are discounts for services that run for a majority of the month - "Sustained Usage Discounts" - link.
Google Kubernetes Engine:
Google Cloud SQL:
Will it be enough for us?
As you can see, you will quickly burn thru $300.00 in credit in only three months.
Take into consideration that Google's goal is not to give away services. The goal is to introduce you to Google Cloud and provide enough services at zero or low cost so that you can complete an evaluation of Google Cloud.
Is it possible, for example, to stop project and billing every night or even stop and start at any time?
No, you cannot start and stop billing every night. In order to stop billing for a project you must delete the project and everything in it. Some services such as Google Compute Engine (GCE) can be stopped. This stops billing for the VM instance but billing for storage, snapshots, IP addresses, etc. continues unless you delete your instances every night and recreate them each morning.
Services such as Kubernetes or Google Cloud SQL cannot be completely shutdown. Technically you could by deleting everything but then recreating everything takes more time than the savings gained. Very good for practice however.