Yes, it's too late to challenge this years assessment, but there's no reason no reason not to start thinking about next year. My clients recently hired an appraised to handle a tax appeal for them. He got their taxes reduced from $18,954 to $11,927! I'd be happy to pass along his information if you are interested.
Your assessment has very little to do with what your home could actually sell for in today's market. If your hoping to get an idea of what you home would sell for in the current market, I would be happy to complete a market analysis for your home at no cost or obligation.