An article by George D. Medlock looks at the question of what period of time is sufficient to be considered an "unreasonable delay" under the doctrine of prosecution laches?
In Symbol [Technologies, Inc. v. Lemelson Medical, Education & Research Foundation Limited Partnership, 301 F.Supp.2d 1147 (D. Nev. 2004)], the district court held that a period of 18 to 39 years was sufficient to invoke prosecution laches. However, as recently as 2003, the court in Reiffin v. Microsoft Corp., 281 F. Supp. 2d 1149 (N.D. Cal. 2003), held that
a prosecution period that spanned a period of 15 years was not an unreasonable delay sufficient to invoke prosecution laches. The lower end of this scale is also unclear. A two-year limitation is probably unlikely, since the patent statutes specifically allow patentees to broaden their patent claims within two years of the issue date. However, since the court has not indicated a set limit of time, it is unclear what period of time suffices to be considered unreasonable.