How long does belviq (lorcaserin) generally take to reach full effect ("kick in")?

At least 1 week: Give it at least 1 week, may vary depending on some other factors. It takes about 3 days to reach steady plasma concentration.

12 weeks : The manufacturer recommends measuring the response to Belviq therapy at the 12-week mark. If a patient has not lost more than 5% of their weight at week 12, it’s less likely that this medication will help achieve meaningful weight loss. So the 12-week mark is a good place to consider Belviq as having achieved its full, likely effect.