hCG levels

How often is it that your levels double? I thought it was like every 2 to 3 days. I took a test Saturday morning and got a faint positive. when should I try and retest? I can't depend on a missed period because I had a miscarriage about a month ago and haven't had a regular period since. I'm having similar symptoms I did when I decided to take the test that was positive then I miscarried soon after.