If you want to find out approximately how many Japanese words you know, the freely available Japanese Vocab Test is for you.
It tests your vocabulary using a two-step system. First, it displays fifty words ranging from extremely easy to extremely hard. Your task is to select the words you know, and then using those words as data, the website generates a second test of one hundred words that correspond to your level, based on its own algorithm. After the second test, it estimates your vocabulary size.
Although the results are not exact, this method (Paul Nation's Vocabulary Size Test) is known to be a fairly accurate method of testing vocabulary size. What's more, it has two word lists to choose from, Wikipedia (〜43,000 words) and Core 10K (〜9,400 words). Advanced learners can choose to test their knowledge based on the Wikipedia list, and beginners can use the Core 10K list to focus their test on more commonly used words.
Rome wasn't built in a day and neither is your vocabulary knowledge. Still, taking this test periodically will offer you a good way to check in on your progress. In any case, it's a fun challenge, so give it a shot!