"I've been counting."

Technically, David is a bit sloppy here. It's not like he would have died if Shepard had not intervened. Rather everybody else would have. He'd have lingered in some nightmarish existence in a VI hybrid state but technically, he'd have remained alive.

*goes back into robot standby mode*

But even assuming that he would have died soon after the events in Overlord (if Shepard hadn't intervened), I doubt he could have predicted exactly the number of days he would have lasted, so an accurate count is still impossible.

Well, being an "autistic savant" is cited as the reason he could communicate with the geth in the first place.  In my ME3 run started just with the comic (i.e. it assumes the player did not do Overlord DLC),  Gavin Archer indicated that Cerberus/he had to kill David after he went rogue.  After that happened, TIM demanded he get another subject "like David" to continue on with the work... and that's what triggered Gavin Archer to leave and go into hiding with Brin's group of scientists.  (Not doing Overlord in ME2 changes the story quite a bit when you talk with Archer.).

Also, I wasn't implying it was common... but early on, movies like Rain Man and other media gave rise to such stereotypes about autism... and it was difficult to convince people to see things more accurately... and, in some cases, more humanely.  Statistically, I was told that "savantism" occurs in the autistic population at no higher a rate than it occurs in people who do not fall "on the spectrum"... so, its incidences are likely not associated with autism at all.

Consider that to for a cancer patient to say "I'm counting the days X treatment lengthened my life" is automatically considered to be a vague statement of appreciation for the extension of life beyond an already ambiguous estimate... and we don't analyze it at all with the expectation that the person stating it is literally counting off each day.  It's also very common for people to say "I'm counting to the days until X happens" and we usually don't expect them to be actually counting but we accept the statement as just an indicator that they are looking forward to that event.  However, in David's case and judging from this thread, some people here do expect that he is actually and actively counting each day.  Why?  Because it plays into a stereotype about autism.