Spending time testing one-on-one with individual people, asking questions, getting feedback. It's so important! It is one of the most important parts of developing a product.

After a while, you'll see diminishing returns. First couple user tests will be the most useful.

Rule of 5. When you have tested 5 different people, you may say "good enough". Don't just test within your office or company, but ask different ages/genders/races/professions/interests/personalities. After those 5, it is still great to test, but no longer mandatory, as most of the issues will have been found.