These are very difficult questions to give absolute answers to. I will give it my best shot though.
The doc and Tommy identify 2 different types of penny stocks for us here: stocks who have significantly dropped in price due to some sort of economical factor and stocks that have all indicators pointing in the direction of gains in the future.
I will start with the stocks that have dropped. These are referred to as having a dead cat bounce take place. This means that it has lost a significant percentage of its pps within a day or two(ish). Quite often these stocks will rebound quickly a fairly decent percentage and allows us to make a quick buck or two. They are highly volatile stocks and dangerous to invest in because it is possible they can become worthless. Quite often this will happen if a company investigates bankruptcy. the rebound in price takes place due to an overreaction of the public to the news.
The second type - the stocks identified by positive indicators using technical analysis. I wont get into describing TA, but the doc has been doing it for years. When the stocks charts indicate a gain is in the future sometimes it happens in a day, a week or months. It is impossible to predict. Fortunately for us if the indicators reverse, the doc will let us know we need to bail out. And it is comforting to know he has money investing in these also.
About the amount to hold - that depends on if you want to be in it long term or short term. Obviously long term you can accumulate more stock because you dont mind selling it off slowly. For short term buys and sales, you need to take into account the amount of volume that the stock has in a typical day. For instance, the volume for JUNI is usually between 20M and 50M a day. That means people buy or sell 20-50M shares a day. If you have 2-4M shares, you can sell them off at once easily. If you have 50M shares, it may take you a while to sell them because there just arent enough traders purchasing them.
One other factor to keep in mind, selling penny stocks you want to be able to sell them all at once. When you make a sale, it has a negative affect on the stock (unless that sale is offset by many purchases). If you are forced to make many seperate sales in a row, you will drive the price down, make your last sales less valuable than your first sales. Sometimes this can be significant.
There are no specific guidelines to follow because it is different with every stock. What I do is watch the ticker for a stock and see how big the sales and purchases are to get an understanding of how that particular stock works and how the price is affected. Hope that helps