Is 3 weeks for a TDEE estimate enough?

Using the adaptive TDEE spreadsheet, my weekly average weight has been 158.0 on the dot for 3 weeks, on 2100-2150 calories. I'm 5'3 and 21F; I don't exercise, but I do housework for around an hour or two per day. Otherwise, I'm super sedentary and only get up to cook, shower, etc.

I'm asking because 2100 just sounds too good to be true, and so does 1600-1850cal/day to lose 0.5-1lb a week since TDEEcalculator.net tells me I'd need to eat 1200-1450 for that, and that's what I've strived for in the past (though always failed).

Is 3 weeks enough for an accurate TDEE? Should I push to 4 or longer just to make sure, or will it really not change very much from what I have?

Also, any anecdotes of people around my stats who've lost on 1600-1800 would be greatly appreciated.