How long does it take (roughly) for a metabolism to slow down from lack of what used to be continuous exercise?

Your metabolic rate will not slow down as a result of you reducing the amount you are exercising, but your weight will change in accordance with your net calorific balance given by calories consumed minus calories burned.

That gives a number that will approximate to your weight change at a rate given by:

weight change per day (in grams) = net calorific balance per day / 9. . (fairly obviously a positive net calorific balance equates to a weight gain, and a negative net calorific balance equates to a weight loss).
. Your Basal Metabolic Rate (BMR) will then be determined almost exclusively by your age height and weight.
The empirical formula found for women is: . BMR = 655.1 + ( 9.563 x weight in kg ) + ( 1.850 x height in cm ) - ( 4.676 x age in years ).

. Weight variation figures change slightly in accordance with some additional variables including: . (a) if you are suffering from an injury (when extra calories are used to repair the injury); . (b) illness (e.g. generating abnormal body heat during a fever); . (c) maintaining normal core body temperature in unusually low environmental temperatures. . It should be noted that the empirical BMR calculation for men is: . BMR = 66.5 + ( 13.75 x weight in kg ) + ( 5.003 x height in cm ) - ( 6.775 x age in years ) .

– Best wishes - Majikthise. .