This can depend on a number of factors, from the current state of the website, to how competitive the industry is, but we typically see results for our clients in around 3 - 6 months.
SEO is a lot like a flywheel - it takes time and effort to get things moving, but once things start to move it usually becomes much easier to continue to build.
Be wary of any SEO agency promising to get you to the top of Google in a few weeks - this is a huge red flag.