I have a lot of experience with WalMart test....let me tell you this and then maybe it will help you
With the first 10 I took I left them sitting about 6-10 hours and a faint line was on there it was infact a false positive (evap) whatever you want to call it, but it was there on all 10 test.....NOW I got my first true positive on the same WalMart test days later...What happened is I was pregnant just really early bc I went in for blood test and it was negative 0 hcg, so 3 days later I took another Walmart test and I swore that at 3 min I saw a very very grayish looking line which I didn't see on the other 10 or so test I took....at 10 mins the line was purplish, then in the morning I took another one and this time it was 13 mins and the line was there although faint I knew it had to be positive because of the past WalMart test I took....Then 3 days later I bought a digi and "pregnant" showed up.....Good Luck and I hope this helps you!!