D.C.
In my experience, supply drops quickly when you stop pumping at work.
What I did (2 babies): went from pumping 2X per workday to 1X per workday (midway through my day) at 10 months. At 12 months, stopped pumping at work and breastfed in the morning and after work. Baby #1 got frustrated with lack of supply and weaned almost immediately at 12 months. Baby #2 stuck with it for another 3 months - I think that my supply stayed a little bit better with baby #2 because he was still breastfeeding a lot at night at 12 months, and baby #1 was not. Then at 15 months, he decided that I was too much work for too little milk, and he'd rather have a bottle of formula. Either way, for me, quitting pumping was, in essence, a decision to wean within a short amount of time.
I was totally ok with this. But if you want to breastfeed longer term, then you should keep pumping twice per day.