It depends.

if minor branding changes mean = changing the logo, putting new labels then its not a job of more than couple of hours (or even lesser than that).

If minor branding = logo change, change of whole skin, layout then it can be a little longer job.

It also depends upon what kind of contract you have with the App developer, like if they cover this "white labeling part" already in the cost or its a new demand from your side.