Agreed with Donnie--mine are usually offered food between the first and middle of March: some will take it that early and some won't (typically the females are the first to take it, but even they won't if they've been bred and have already ovulated). But most all of mine are back on feed by April (unless they are gravid females).
For my larger balls (3,000+ grams), I don't really sweat 200-400 grams of weight loss.
For my big balls (2,000-2,999 grams), I don't sweat 100-300 grams of weight loss.
For smaller ball pythons(800-1,999 grams), I don't sweat 50-200 grams of weight loss.
Granted, all of these weights are over the period of the entire winter, not just a week or two. Any huge amount of weight loss all at once should definitely be monitored closely, and even a visit to the vet if subsequent weighings also show another significant loss. Bear in mind that if your snakes voluntarily refuse food, you might consider dropping the tank temps about 5 degrees, so as the keep the weight loss to a minimum until they act like they want food again (they typically stick their heads out of their hides around feeding time/day when they are hungry). Since their metabolism is based on their temps, cooling them a bit slows their metabolism, and thus they don't lose as much weight. However, if they've had any meals in the past 3 weeks, don't cool. Once they've refused meals for 3 weeks, it's OK to cool them. If you cool earlier, they might not properly digest their last meal.
Another trick you can try is lowering their temps for a few weeks and them bump them back up. It can trick them into thinking spring has come again and that they should begin feeding again. However, balls are the finickiest eaters, so sometimes even this doesn't work for them.
I've read somewhere that most weight loss during the hibernation months is water loss rather than fat (based on a slightly cooler tank temp). I try to weight mine every 2 weeks to determine if there's a huge amount of weight loss, or if it's just typical water loss (which is generally around 25-75 grams per 2 week period, and depending on the size of the snake).
Hope this helps!