Bfp over a few days

Just a question about hcg levels and what's considered good and normal. I had a miscarriage last month and didn't get any super dark lines or strong positive tests at the docs. So I've gotten 3 bfp over 3 days. Yesterday and today look about the same but they are definitely darker than first day. My question is, at 14 dpo (I can only assume when I ovulated, I haven't had a period between miscarriage and now) when should my lines start getting darker? Does it take a few days for levels to go up?