Pumped Milk: Benadryl

Need Advice on Weaning from Breast-pumping

I've been back at work since my son was six weeks old, so I have pumped exclusively since then (no direct breastfeeding.) He's now four months old, and I have to have breast surgery so have been advised to wean him onto formula. I have gotten down to only pumping twice a day (morning and night), but how do I completely stop the milk production? How long should it take me to completely dry up? What should I expect? Any advice would be great. Thanks!!