Cooking turkey breast
You don't judge cooking time. You measure it with a thermometer and get it out when it is done, as indicated by internal temperature. 

All estimates of cooking time based on weight are not precise and will frequently lead to over- or undercooked meat. If you insist on using such an estimate, it doesn't matter much if you use the weight before or after marinating, as your error will be larger than the variability caused by weight difference. Choose whichever weight you want to.