Bob Hobden wrote:
If they aren't Blight resistant varieties then a couple of days before the
plants turn totally black and collapse IME, even if Blight resistant they
are on the slippery slope and will only last another week or two. Even the
tomatoes that look fine will probably have blight and it will often show
after you pick them. Personally I'd pick off everything I think will ripen
and get rid of the rest and the plants.

IME they often hold out a lot longer than that. Mine on the allotment
(all volunteers) started showing signs weeks ago, and I only gave in and
pulled them all today. At the time they were all totally green, now I have
a huge tray of green and the occasional red, and a good few on the turn
which I think will make it.
(it seems to be the round ones that go blighty later from green, the other
shapes seem to get blight early on and once they're picked if they haven't
got it, they won't. Of course, no idea what any of the varieties are -
one is long and sausage shaped, and not at all like anything I've ever
grown in the past!)