Yes, I think there are times when you do stop praying for a person. Or at least there are times when your prayers may not be as intense. We naturally stop praying when the circumstances change (thus the prayer is no longer needed) or the prayer has been answered (perhaps differently than we expected but answered nonetheless) or when God lifts the burden from our hearts. This last category is certainly personal and no one can dictate to another person in this area. God has often laid certain people on my heart for a particular period of time. Generally I will never know why. But I pray for them as long as God brings them to mind.
Remember that prayer is gift, not a burden. We should pray for others as a joy and not because we feel that we have to. If we truly believe in the intercession of the Holy Spirit (Romans 8:26-27), then we can believe that the Holy Spirit sometimes inspires us to pray for others. We should never ignore those moments when we feel led to pray for someone.
See my sermon What If for more discussion of this topic.