It may take longer than 50 or it may take only 30. Considering that I went from zero to private in a R22 in 35 hours, anything is possible. It just depends on how long it takes you personally, and how condensed your training will be. If you were to do the training over 4-8 weeks it will probably take less flight time than if you spread it out over a longer time period. Choice of school will be important also. The fact that you have a commercial cert already will only speed up the ground school part, although there may be a negative transfer of skills in the flying end of things.
I also have a buddy who done a commercial add-on in a R22 in 50 hours. So 30 hours for a private shouldn't be too much of a stretch. Where are you based?