I had a recipe for roast potatoes that states to parboil for 5 minutes before putting in the roasting tin. Unfortunately I don't have the recipe still with me for reference but this is a general question for when instructed to parboil for a short time. Generally speaking, when does the 5 minutes actually start? If I bring the water to boiling point, then put in the potatoes, they cool the water, and it can take nearly the 5 minutes just to get back to boiling point. So should I start timing as soon as the spuds go in the pot, or should I wait until it's boiling again, and then another 5 minutes? 
A good recipe should have guidance on the desired end state you are aiming for, and experience with cooking will help you know what to aim for too. In the case of roast potatoes, the aim at the parboiling stage is to get to a point where a knife can easily go partly into the potato, so that they are partially cooked and to help the outsides get roughed up in the fat before roasting. This will take different amounts of time depending on the potato size (and probably also their variety). Prod them with a knife to see if they are ready rather than relying on a timer. 