That functionality it's already in Odoo. You could use it simply by creating events in the Calendar at the Messaging menu. Those events need to have a reminder alarms set in the Reminders field to be able to launch a popup remainder at a defined time before the start datetime of the event arrive. The alarms time remainders could configured in the menu /Settings/Technical/Calendar/Calendar Alarm.How it works:

In the calendar module there is an extension for the WebClient js class that add an interval based recursive call every 5 minutes to the url "/calendar/notify" to poll (like a cron) for events remainders to be displayed for the current user. 

To use that you could just create event with remainders alarms of type notification in your crm follow ups manually or dinamically by code. If you need to do something similar but not exactly that you could see how it's developed at /openerp/addons/calendar/static/src/js/base_calendar.js, but I think that it's enough for every remainder scenario and like always, it's better be integrated with the core than having to maintain our owns modifications

Hope this helps