We don't know the answer to that question, but scientists have studied the region of the 1964 Alaska earthquake for clues as to when large earthquakes have occurred in the past. In 1964 very large vertical displacements were caused the the earthquake. Plafker, Lajoie, and Rubin (1992) report that "Pre-1964 vertical displacements in this same area are recorded in the coastal [geology]." By studying the history of these vertical displacements, these geologists have developed the time sequence of previous events. Their conclusion is: "The tectonic displacements related to the March 27, 1964, Alaska earthquake and the [evidence of past earthquakes based on old shorelines] in the same region demonstrate the complexity ... at the eastern end of the Aleutian arc. These data indicate: ... [there has been] at least 800 years since the last 1964-type earthquake in this same region....[and the] intervals [between previous] ... great 1964-type events [varied from] about 600 to 950 years."
Another approach to this question is to figure out, given the 56 mm/year rate at which the Pacific plate is colliding with Alaska, how long it would take to accumulate the strain that was released in 1964. This earthquake released on the order of 20m of slip over a area of 750 km by 200 km (Page and others, 1991). Twenty meters is 20,000 mm. Dividing 20,000 mm by 56 mm/year gives 357 years for the time it would take to accumulate this much strain. That is a minimum estimate, because some of the plate motion is taken up by smaller earthquakes and some slip may occur without earthquakes (called aseismic slip).
Both of these lines of evidence suggest that it will be hundreds of years before we have a repeat of 1964 earthquake. However, this is not to say that there couldn't be much smaller but still very damaging earthquakes in this area of Alaska. For example, both the recent Northridge, California, and Kobe, Japan, earthquakes were less than magnitude 7.
References
Plafker, George, Lajoie, K.R., and Rubin, Meyer, 1992, Determining recurrence intervals of great subduction zone earthquakes in southern Alaska by radiocarbon dating: in Taylor, R.E., Long, Austin, and Kra, R.S., eds., Radiocarbon After Four Decades: An Interdisciplinary Perspective, New York, Springer-Verlag, p. 436-453.
Page, R.A., Biswas, N.N., Lahr, J.C., and Pulpan, Hans, 1991, Seismicity of continental Alaska, in, Slemmons, D.B., Engdahl, E.R., Zoback, M.D., and Blackwell, D.D., eds., Neotectonics of North America, in the collection The Decade of North American Geology Project series, The Geological Society of America, Boulder, CO, CSM V-1, p. 47-68.