For a dynamic site, it's roughly 60-90 concurrent users dependant on your host and the server load. However, I suggest you install super cache on your blog as that will mean you can use shared hosting for longer.
At the rate you're growing, Just go ahead and get a VPS....
I'd say it's hard to judge simply based on a unique visitors amount of even concurrent users amount. The best judge of if you're to much for a shared hosting environment is if your hosting provider tells you its time to move on. There's also a lot of other things to factor in such as if the content is static or dynamic. wp-super-cache can help a lot with word press blogs.If it is dynamic, how completed are the scripts you're running? Do you have a rather large MySQL database that my cause additional load on the server if not indexed?
All things to take into account...
It seems as if you're ok if your hosting provider hasn't told you its time to upgrade yet, but at the rate you're growing, it's only a matter of time.
In my opionion the move off shared hosting depends on finances. If your site is just a hobby and doesn't provide an income then staying on shared hosting until your host says otherwise is usually what happens. But if you are making money from a high traffic site it just makes sense to go dedicated to protect your own interests.
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