Well I had it from about 15-16 weeks (when I squeezed), but when I mentioned it my midwife looked at me like I was some sort of weird alien milk machine or something, and said that for most woman it happens during labour or at birth, not during pregnancy, and if it does then usually in the last 8-10 weeks.

Same here. Woke up wet one morning and wondered WTH was going on. That night when i was reading a book I felt it on my arm and when I sat up i leaked. I was leaking everywhere.

No, it is not an indication of how much milk you will make, but in some cases it may help your milk come in earlier. Mine came in late Day 1.