What is the threshold of trust for search?

Assuming that you have to find what you are looking for x % of the time in order to trust a search service and become a habitual user of said service:

What number is x?

Links to research papers would be sincerely appreciated.

E.g. is there any officially available source for how often people find what they are looking for using google.com and related services? Are there regions (e.g. geographical) where findability is lower, and is it documented how this decrease affects customer loyalty?

Also, is there a documented region between y % and z % where trust is accelerating, and every additional improvement has substantial ROI (defined as increase in recurring use of the service)?

Finally, are there any enterprise search vendors that have stated how often their clients' employees should expect to find what they are looking for? E.g., have anyone heard sales people from Microsoft pitch such figures for SharePoint?

Background for the question: I have worked on enterprise search solutions for 12 years. Especially for "out of the box" SharePoint deployments, my takeaway is that many employees don't use the internal search for lack of trust, since findability is somewhere below 25%, sometimes even reaching the single digits (in stark contrast to the experience they have using google.com). To encourage owners to invest further, I assume it would be effective to show them where they currently are on the graph below, and at what point they can expect their employees to start trusting the service.

In this graph, "SharePoint" is understood as the product's default use case; "a document management and storage system" [W], where there is little or no editorial maintenance. "Intranet" is understood as the corporate site for editorial publication of internally relevant articles and tools.

One client states as a success criteria that "60% of the time, people should find what they look for". However, they do not have any data to back up that this is enough for people to start trusting the search. In my previous experience, if you can provide a findability of 75% the users return to the service, and vice versa, if it is lower that 30% then they shy away. But my sample of users and services are low, and I wouldn't refer to it as actual research. Also, the data points are too few to be of much value - you don't really need any data to infer that 75% is quite good and 30% is really bad.