You can test as often as you like, but you don't NEED to. I usually let my beer set in primary for three weeks. I then test when I take it upstairs for bottling kegging. If it is close to expected final gravity I bottle or keg. There is some risk doing this, since I don't take multiple readings, but over time I have realized that if it is where it is suppose to be there isn't going to be much changing after 3 weeks, as long as there was some krausen within the first few days. I just shine a light through the bucket and you can see the outline of the krausen. Over 5 years I have only had once beer that wasn't ready after 3 weeks.