SEO can generate traffic in 3-4 weeks. However, there are many variables that can affect the time it takes to generate traffic through SEO.
Site content (quality)
Backlinks
Targeted keywords
Competitor rankings etc.
SEMrush offers a free trial for those who are new to SEO. They offer a powerful platform that will let you monitor all aspects your SEO campaign. This includes competitor research, backlink profile analysis, top pages, local listings and organic traffic stats.