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?