Ignoring the exclusive aspect (should have a date certain), if you go to querytracker.net and look up agents, you can see what their average response time in the statistic section. If you go through my experiment with a query, you'll see that most of my responses on partials (very limited sample size) are within a month, but I do have one that I have never heard back on.
With an exclusive, you're definitely within your rights to contact the agent, and if you don't receive an immediate response, to start sending out queries, IF YOU DON'T HAVE A DATE CERTAIN. I'd double check all of my correspondence on that issue.
Best of luck,
Jim Clark-Dawe