How often should I adjust my calorie goal as I lose weight if I want to keep losing?

Your TDEE (maintenance calories) reduces by approximately 5 calories for every pound lost.

So you already get 5 x 45 = 225 calories less per day, based on your current weight loss. Or put another way, you are already losing almost 1/2 lb per week less than when you started - eating the same # of calories.

So yeah, in theory you would have to reduce your calorie quota to continue losing weight at the same rate. BUT ... eventually you will hit a wall, where if you reduce your cals further, you'll either be below a safe minimum (1500 male, 1200 female) or more generally, just not be getting enough food to feel satisfied and continue with successful weight loss. This is why it's a good idea to really think carefully about whether you want to push on at the same weight loss pace or reduce it, as you lose weight.

From my experience you are MUCH better off moderating your goal than reducing your caloric intake. The latter is really a challenging way to go about things, and ends badly for many people. I've had good success gradually reducing my weight loss goal so as to keep my calories more or less constant. In fact my calorie quota has gone up since I started dieting. 90 lbs lost.

What is your recent rate of weight loss?
I'd be more concerned that your rate of loss is appropriate for how much weight you now have to lose.

It's time to look forward to how you are going to handle the transition to maintenance and how fast you lose the last few pounds can make that harder or easier.

I wouldn't even waste my time with mfp calorie goal. For one, it won't let you set less then 1500 calories so if you want to lose 2lbs a week, its useless.