I normally have mine replaced at nine months. This time I had to let it slide in the lead up to my GRS so I've gone almost 12 months now (due a replacement in one week, yay). At the 9 month mark the endo said I'd normally be due a top up, but as I was just on 800pmol/L (I go to the same endo as AnonyMs) I'd be able to glide through on reserves for a while. The thing about the implant is that it doesn't just run out, the let down is probably fairly gentle.
During months 9-10, I can't say I noticed anything obvious... possibly a little lethargic and a bit irritable... but that could have been the stress of work and in the lead up to GRS. Towards the end I was experiencing headaches, something I don't normally get. Given I was also coming off Spiro and progesterone in month 10 it could have been anything. Other than that, I remained in pretty good health and spirits.
For month 11, I'm back on the oral progesterone now and using an estrogen gel to tide me over until my implant. I have to say I feel fine...blood test tomorrow, will be interested to see my levels (especially T) post-op.