It was noticeable within the first 6-8 weeks, honestly. By month 2 I was seeing readings in the low 120s/high 70s consistently, which was lower than I'd been in years even on medications. My PCP said the early improvement likely reflects both the natriuretic effect and reduced caloric intake affecting fluid balance.
But the real BP improvements correlated with substantial weight loss. The biggest drops came between months 2-6 when I was losing the most weight. After that it stabilized.