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.
Might as well go ahead and pickup a VPS.