depends on kind and quality of emulsifier used. Note, that all emulsions are thermodynamically unstable.
There is a phase inversion point when oil-in-water emulsion transforms into water in oil, which is determined by emulsifier, ratio oil/water, temperature ..
Further, if you add oil slowly into water under agitation phase inversion point (ratio) will be higher than if you start with a water in oil emulsion by starting adding water into oil.