Like many behaviors that can aid in weight loss, this answer will vary from person to person. Some people really hate to get on the scale. And the answer for them would be "you never have to". But most people find it encouraging to see the numbers go down and this keeps them plugging away at keeping up their healthy eating and exercise.
It does get frustrating if the numbers don't go down quickly enough--and if you're looking to lose 9 pounds this week, you will be very frustrated! The typical recommendation is to weigh weekly and expect to see a one or two pound loss. I find it helpful to keep a graph and track weekly weight loss, because the pound or two lost each week is much more meaningful at the end of the month. When your graph shows 5-10 pounds lost, frustration easily turns to encouragement!
Some people--and this is true of many who have lost weight and kept it off--weigh themselves daily. Yes, that's right, every day. If you do this, it is very important to know that daily weight fluctuations are not the same as fat loss or gain. A person's weight can change by 2-4 pounds each day based on water retention, as an effect of sodium intake, after eating a large meal, etc. Then why weigh every day? It helps you learn that weight is a just a number; that eating a large meal can't really mean you gained six pounds of body fat; and that if the number keeps going up it's tme to change something right away. Frequent monitoring helps people see when they need to adjust their diet so they can maintain their rate of loss, or maintain the weight they are comfortable being.