What is the limit of time resolution in fMRI?
The quick and dirty answer is: fMRI doesn't perform well in the temporal domain, but excels in the spatial domain among non-invasive imaging methods. If you need an answer as to where the action is, use fMRI. If you need answers to temporal questions, I recommend using electrophysiology such as EEG, which excels in the temporal domain, or MEG for a combination of good temporal and spatial resolution.  

As to what the limit exactly is, I would say a few seconds. fMRI measures blood oxygentaion responses, (See the answer on the question: "What does fMRI measure exactly"), which are inherently slow. Neural processes in local neural networks occur in milliseconds, in larger networks in the order of hundreds of milliseconds. Blood flow differences, in contrast, occur in the order of seconds.