Firstly, you can use a number of techniques to plan your tasks. One of my favourites is making a process map and using it to produce a Gantt chart; Wiki should show you everything you need here. In terms of how to determine how much time to dedicate, ask yourself how much the project is worth, and what other tasks you have to do. Give everything a priority, work out how much time you have to do the alloted tasks in, and how long each task should take you (go for worst and best case estimates and do it for both), then you should have a clear idea of how to divide your time. Spend a little time planning and everything should fall into place.
Is this what you were getting at?