The only reason to be on PPD is because you made a mistake and now that you are on it you made some sales and you are at 70/80% +. This happened to me a couple of weeks ago and I only changed it back when I was at 60% and decreasing daily. If you are at 50% or less, just change it to PPS as fast as you can.
Answering your question, it depends on many factors, like the sales/renewals you make and the mb's you upload daily, to give some examples. You have to make sales constantly in order to increase those 2%; for example, if you have an one year sale, you probably will be at 40/50%, but you'll lose massive cash since you won't get anything from those 40$ you could have received; and in order to increase your rank to 80% in the next day, you'll have to make another big sale, you'll lose even more cash and so on... Snowball effect.
Here's an example of my two first days uploding (never used any host before Oron):
At PPS you won't see 0,01$ entering your account every day, but you can see 24$ all at once. Even if you are noob, like I was at the time, and probably still am when compared to most guys here.