How long is a test still accurate?

I'm a week late for my period. This isn't unheard of since I have pcos but since my fibroid removal surgery in December and starting metformin they have been pretty regular.

I just got a generic Walmart pregnancy test on a whim (felt like I should). I took it Wednesday night . TMI but I didn't pee very much so I felt like there wasn't much stream on the stick. There was no line, I thought I could just make out a hint of one but decided I was trying to see what I wanted to. I tossed it on the top of the trash bin and went to sleep. In the morning I noticed it now had a faint but definite and clear line!

My question is would this count as a positive or does it become invalid if it sits for several hours ? It took a long time to show up so I'm wondering if it's just what all tests do after being discarded?

I plan on taking a first response one a week from now if I still haven't had my cycle. I'm trying not to get my hopes up since we miscarried a couple years ago but I just have a general feeling that I am, like an instinct? I've also had an upset stomach for a few days now , thought I had mild food poisoning but my husband thinks it could be related since I was a bit queasy this morning.

Anyone else have experience with looking at a pregnancy test way after the fact??

Thanks and sorry for the lengthy post.