It is not easy to give an accurate picture as there are many variables that affect this. At current difficulty, ROI would be achieved in a month. But three months into the future, difficulty will be higher. How high exactly depends on a number of assumptions.
e.g. organofcorti.blogspot.com forecasts that it will increase about 7 fold by end of July. In that case, ROI will take about six months. With overclocking, it will be four and a half months. He of course assumes that Avalalon batches 1,2 & 3 will have shipped by then, BFL will have shipped and ASICMiner will also have about 100Thps by then. If any of these assumptions do not materialize then difficulty will not increase 7x and ROI will be achieved much earlier.
In short, timing of essence. Miners who get in early will get a better ROI than those who wait.