Normally speaking it shall be easy to find, for instance, the family card of your great grandfather in Dutch archives. Other researches are less straightforward. Looking up such a family card also does not take much time. In order to answer other questions you sometimes need to go through piles of documents.
When dealing with more complex questions we usually start with doing a short inventory research of one or two days. In it we find out where to look for the answers and estimate the chances of something being found. In further consultation with you it can be decided to do further limited and well-defined research.
This follow-up research may again be rather straightforward, or not. Naturally those sources that promise the most will be looked at first. And then those which offer less chance, or cost more time. Etcetera, according to your wishes of course.
Sometimes one must resign oneself to the fact that an answer cannot possibly be found (ever more so when going back further in time). If the most obvious archives have produced no result a research can look ever more like looking for the needle in a haystick, without even knowing if it really is in there. That is why, always in close consultation with you, the strategy of which paths to follow will be discussed. Agreements can then be made about (follow-up) researches of usually one, two or three days.