How long will it take for the ocean to erode Rockall?
Impossible to give an accurate answer because nobody has ever studied rates of erosion of this kind of rock in such a setting. Typical long-term rates of cliff cut-back in an exposed position are 5 to 20 cm per year, but that is for average rock, which is much softer than Rockall. This is a complex rock, mainly comprising Aegirine Granite, and containing some unique minerals. I'm guessing that the combination of sea salt corrosion, frequent wetting and drying, and chemically aggressive leachate from all that guano will result in a rate of chemical erosion of 0.5 to 5.0 mm per year. Let us say 2 mm per year - a high value but under the extreme circumstances, certainly plausible.  Given that the rock is only about 25 metres along the lesser axis, and that chemical and weathering attack will occur on all sides, that should give a 'demolition rate' of about 6000 to 7000 years. But that's only chemical erosion. There is also the physical erosion to consider. None of the photographs I have seen give a good impression of the potential for basal abrasion. What your photograph does indicate however is that there are eroding oblique fractures. These will facilitate sea water and wave ingress which will eventually fragment the rock and accelerate its demise. Overall I would be very surprised if there is anything left above the waves in 4000 to 5000 years from now. Even without erosion, climate change is likely to raise sea level at least 7 metres by that time, in which case it may completely disappear in as little as 2000 to 3000 years.