I am just guessing about the reason behind this question...
Sometimes functional consultants need to provide a rough estimation for a change request, which may or may not be performed in full by the same person.
Unfortunately this is not an exact science and in addition to time for the specific customizing activity you need to add the time for updating the project documentation, potential handover sessions, administrative tasks for processing the transport request, support for QA testing, etc.
It could happen that these additional and often underestimated tasks take 80-90% of the total time to implement. So in case you are the person that has to calculate this time, make sure to consider them as well - otherwise you risk to be way over budget and not deliver on time.
If you are new to the company, it can be useful to check how long it took your colleagues to implement similar change requests - at least for the administrative part.