My prayer time is the hardest thing to deal with manly due to my work hours. Which is over nights central standard time USA. The problem is since we have a relationship with a living God. The contrast of having a set time with Him as a discipline, rather than the spontaneous praying, at times can be challenging. What I do is both, I use a set prayer time as well as talking to Him about everything. In addition, I journal recording where I stop in my reading program and if I mess a day I pick up where I left off.