Unfortunately, this is one skill that probably can only be developed through repeated mistakes because it is not likely that a freshly minted graduate will be able to give an accurate estimate of the duration that an activity will require.
If anything, it is common for people to overestimate their abilities and underestimate the amount of time required to complete an activity. As a rule of thumb, you will probably need to add a buffer to your estimates but not to the extent that you end up “padding” your duration estimates (which is not considered professional behaviour).