I assume you mean approve their registration after x days automatically? That would be doable using CB Auto Actions.

CB Auto Actions can be ran using CRON Tasks configured in your hosting platform. Using the Internal Users trigger and its URL you can pass an auto action through your entire user base. That's where you'd do this processing. To activate them you'd just call the built in API for it. For example the below Internal Users usage should work fine for this.

Global
Triggers: Internal Users
Type: Code
User: Automatic
Access: Everybody
Conditions
Field: Custom > Value
Custom Value: [cb:parse function="time"]now[/cb:parse]
Features: Format Functions
Operator: Equal To or Greater Than
Value: [cb:parse function="time" time="+3 DAY"][registerDate][/cb:parse]
Features: Format Functions
Code
Method: PHP
Code:
The above for example should automatically approve their registration (and enable them if they're confirmed) 3 days after they register. You'd either need to run the Internal Users URL manually daily or setup a CRON Task to wget or cURL GET load it. This is an advanced usage though and don't recommend it unless you've become more familiar with how CB Auto Actions works.

1. the PHP code sets the approval to true but I whish to set activation also. What's the code fot that?

Assuming the user is confirmed already it will handle marking them approved and enabled with that single function call.

2.I am not famliar with crontasks. But I realise that wget or cURL sets the link to the joomla cb action.
But what should be the full statement for that link?

wget and cURL examples can be found below.

www.joomlapolis.com/forum/277-cb-paid-subscriptions-support/237481-cron-job#297853