OmniFocus Standard (and Pro) have an existing field for estimated duration, but metadata support is something
we plan to add to OmniFocus eventually.
(One of the design goals for the v2.0 release of OmniFocus for Mac is no changes that would require customers to upgrade their iOS apps. That means no new database fields in that release, but perhaps a bit further down the line.)
Adding this post to the metadata feature request we have open. Thanks!