How long does it take for the HCG to build up?

2 days ago I took a dollar store test, it was a BFN. So this morning, I went to Walmart and went ahead and got the first response (2 pack). I only took one, with morning urine. I def. see a very faint line, but I'm not sure if it's an evap or not. It came on the screen within 2 mins and hasn't faded any. If it was indeed a faint positive, how long should I wait to retest for a darker line? Like how long does it take for the HCG to build back up? I spent like $10 on two, so I don't want to take it too soon.

48 hours

When I get my blood work done I have to come back in 48 hours for my numbers to double.

Quoting InkDMomma:" 48 hours When I get my blood work done I have to come back in 48 hours for my numbers to double."

  dag, 2 days! I'll be tempted all day tomorrow lol. Thanks!

Wait a couple days. I took my first test the day I was supposed to get my period and the 2nd line showed up instantly but it was faint. I waited 2 more days took another and that time it was really dark..I took another one the next day just to be sure lol

I take tests every other day, because everywhere I have read says 24 to 48 hours. Meaning they should have doubled by two days.
I took one yesterday (BFN) and plan on taking one tomorrow.

If you got a FRER they are 12.5 miu. Dollar stores are 25, so it is possible! I saved my FRER for tomorrow!