It is often very useful to have a facilitated workshop for your first RBV estimation. This workshop will take your backlog of epics, features, or initiatives, create an affinity sizing map that very quickly creates value estimates for your items.
Often, a hundred or so can be done in an hour or two and it even solves the problem of comparing the value of a business-oriented item to a technical one (e.g. tech debt remediation). Its duration depends on how many epics, features, or initiatives you have, and how many stakeholders participate.
Agile Accelerate 