yeah that's stupid. When I test I throw it out after a few minutes and no line. I figure even if the line COULD still appear in the next 7 or so minutes before the test is invalid, it probably won't if it's not there in 2-3 minutes. I'll just test again later in a few days.
Not that this has ever happened to me, but if I did see a line appear a few hours later, I would test again the next day to make sure the line is there within a few minutes. It should get darker each day (assuming similar concentration of urine) so if there is any uncertainty, just test later.