When can a plaintiff rely on estoppel against the assertion of the statute of limitations?

When a plaintiff " 'relies on an estoppel against the assertion of the statute of limitations, the sufficiency of the allegations of estoppel may be tested by a general demurrer. [Citations.] Once it is determined that the elements of an estoppel have been sufficiently pleaded, however, the question whether the statute of limitations is tolled by the conduct of the defendant is one of fact which should be left for resolution by a jury and not determined upon general demurrer.' " (Muraoka v. Budget Rent-A-Car, Inc. (1984) 160 Cal.App.3d 107, 117, italics added.)