Developing the Board
Exposure times for the PCB vary depending on the exposure method. For instance, if you're working with a single sided board and plan to develop using the overhead florescent lights I recommend about 15 minutes of exposure. Before peeling off the tape and exposing the PCB, make sure to have a flat plate of glass such that you can lay the transparency over the peeled board and then lay the glass over the transparency so that it is flat against the PCB. It is helpful to dim the lights during this step to minimize exposure until ready. Once exposure begins do not move the board.
If you are building a double sided board, it will be a good idea to use a uva light chamber so that you can expose evenly on both sides. If you do not have one here is a link to construct one using leds.  Another method to expose a double sided board without a light chamber is to secure the transparency on both sides of the board and then expose each side individually for the same amount of time.
Expose long enough such that all of the uncovered copper goes away but not so much such that the protected copper goes away. Note exposure times in a UVA chamber will be much less than 15 mins.
Once the board is necessarily exposed, develop the board by swishing the board around in a solution of NaOH. This should be done until the exposed portions (where light was able to hit the board) are clean copper. There should still be traces where the copper will be on the final product.