How long does it take to write a good hand when practicing calligraphy for the first time?

It depends on your original foundation, your understanding of calligraphy, your knowledge, and your investment in practicing calligraphy.

Generally speaking, practice the word 100 at least every day, persevere and get started in three months. It will take at least half a year to write it. That is, to reach the primary level. If you have good comprehensive conditions and are willing to work hard, you may reach the intermediate level in half a year.