Time estimation of user story can be done by using the Planning Poker technique. In this method, the team members involved in the project are given a set of cards with numbers representing time units (like story points which can be converted to hours or days later). Each member privately selects a card that they think represents the effort required for the user story. Then everyone reveals their cards at the same time. If there are big differences in the estimates, the team discusses the reasons behind it. This helps in getting a more accurate time estimate as it involves multiple perspectives.
One simple way is to break the user story into smaller tasks. Then, based on past experience, estimate the time for each small task. For example, if a task is similar to something you've done before that took 2 hours, you can estimate it as around 2 hours.