Project plan in agile projects

Probabilistic Forecasting might be of interest to you. There is also a book to help: When will it be done? by Dan Vacanti. Your team will have to start looking into Throughput, Cycle Time, and Work Item Aging metrics.

Here is a blog that may help you: https://www.scrum.org/resources/blog/create-faster-and-more-accurate-forecasts-using-probabilities

Should we follow typical traditional project tool based project planning ( like weekly, monthly ) in parallel to agile planning. 

What matters is where the plan comes from and the extent to which it is validated by the empirical evidence of delivery. Scrum is predictable rather than predictive. Bottom-up intelligence is key. The shorter the leap-of-faith being taken the better. 

When my CEO asks me, when the project will be completed (roughly) , without sprint planning done for entire back log, how can I share the plan?

Congratulations on having access to your CEO.

Offer to do better than that. In Scrum each Sprint is a project, and the deadline for completion is not rough but absolutely clear.

Invite the CEO to the Sprint Review as a valued stakeholder. He or she can then help to ensure that the right thing is built at the right time, using their position to remove organizational impediments to the team's progress.

Thanks Chris and Ian Mitchell for the comments.