What activities should be done for installing Apigee X Enterprise, including validation proces?

Well you can do it quickly if not for enterprise-grade use. But if you want the result to be enterprise-ready, then you will probably need longer than 8 hours. Other people, other teams will be involved including networking infrastructure, security, I&AM people, architecture, ... Aligning all of those teams takes some time and discussion.

You first need to "onboard" to Google cloud. This involves setting up identity, a billing account, a project folder hierarchy, RBAC, and other foundational work. Checklist here.

After that you need to plan your Apigee configuration. Which regions? What will you use for the organization and environment structure? What will the network connectivity be? VPN? Interconnect? (Be aware: you will need a /16 CIDR block for your VPC network, for each region. Sometimes that presents a challenge.) After you make those decisions, setting up Apigee is relatively easy.

In some enterprises, when the various teams are engaged in other projects too, all this can take weeks.

In contrast, when I set up my own "demo" environment for Apigee X, it took less than an hour. I skipped all the steps that would be relevant if I were operating in a larger organization!