I dunno, mine was gone gone gone by about 15 weeks last time. I couldn't express anything, never heard ds swallow until colostrum came in around 8 months. However, my ds was never offended and never stopped nursing. I don't think it's invariable, but it's pretty common to have a steep drop in supply or complete drying up - like your body needs to reboot and start the cycle again, to get the colostrum and newborn milk in again.
Check out "Adventures in Tandem Nursing," I'm pretty sure there was more concrete information in there.