Weekly works for me. Like Micheal said it is important to have a routine.
For example, I weight myself every Friday morning before my workout. On Thursdays I try to have a fairly consistent, low sodium diet. One salty meal could add 2-3 lbs of water weight the next day. I try to get the same amount of sleep every Thursday night. On Friday morning, I try to always get up about the same time and go through the same routine every week. I always have a banana on the way into my workout and will get onto the scale after I get my workout clothes on before I put on my shoes.
It's amazing what a little change to the routine can make. One week we had a going away lunch for someone at work. We went to a restaurant. Like I usually try to do, I had looked up their menu before had and knew what I was getting. I had even called ahead to find out their serving sizes. The one thing you cannot control is how they season their food...and like almost all places, it has lots and lots of sodium. Long story short, I hopped on the scale the next morning as was up 2 lbs....which at the time was a shock since I had been consistently dropping 1-2 lbs or more every week. Well the next week I was down 5 lbs. All that extra weight was water as a result of a high sodium day.
Point is, be aware of tricks the body plays when weighing and don't let a bad scale reading throw you off.
Male 45, 5'6"
To date - 32.6 lbs down
Start: 20 Oct 2011: 210.2lb
, body fat =35.3%
Original Goal: 20% body fat - achieved 3/1/12
Last weigh-in: 3-May-2012: 177.6
Last body fat measurement: Oct 15,2012: 14.1%
Goal: Maintain < 20% body fat and < 180lb
Days Maintained : 427