Determining the events you need to track is very much a product thing and is something that is best done by someone with deep product knowledge (usually product manager + lead engineer).

Here are some things to keep in mind:

1. Start small and focus on getting the infrastructure right, then you can scale and iterate.
2. The real challenge comes from consuming the events, not firing them. When calculating implementation time, the biggest question is... how many custom consumers do you need to build?
3. Depending on the size of your application, it can take anything from a few hours to a few weeks. Most teams will get it done in a few days.

You can use third party tools (like Segment or AWS SNS ) to record your events or you can record them yourself. If you decide to record them yourself, you will need to have a job queue and workers that then process the events and notify any event subscribers.

I highly recommend you use a third party service - save yourself the devops headache.

1) In today's time data is the new oil so, you should track every single action/event that is out there in the system. Its okay to have more metrics than to have to no metrics.
These events should always be stored in an append only database, you can get more clarity on this approach by recommending your developer to checkout Event sourcing.

2) Time will depend upon your application and different modules. It can go from 1 day to 2-3 months or more depending on the size and variance of the application.

3) There are multiple paid tools but if you are an open source enthusiast you should checkout ELK stack.