There should be a field for entering a requirement estimate, just as you can for an overall feature. The sum of all requirement estimates should roll up to the feature.
|Release time frame|
When enabling capacity you may choose the default setting so that you can enter estimates at the feature or the requirements level. If you choose requirements, any features that do not have requirements can be estimated at the feature level.
If you want to change how a feature is estimated, you may do so through the Actions menu or in bulk through the Bulk edit menu on the Features List view (you must check at least 1 feature to show the Bulk edit option).