A very difficult question to answer without first performing specific SEO keyword research. Different rankings and keywords have different difficulties and competition.
The higher the competition, the longer it takes to gain rankings for that search. Then for less competitive searches, the quicker it is to rank.
However, competition is usually based on how much traffic a keyword or search term generates. The higher a keyword’s traffic, the more competitive they tend to be. Vise Versa, the lesser the traffic, the less competition there usually is.
Working with a specialist SEO agency, an initial strategy will generate 3 groups of keywords. Easy, Medium & Hard. Then each month, keywords for all 3 groups will be targeted, allowing for short, medium and long-term return on investment.
Short-term, easy, low traffic and low competition keywords can show ranking results from as early as 3 months. Whereas long-term, hard, high traffic and high competition keywords can take 1-2 years+ before a top 3 position ranking is achieved. It all depends on the target search term/keyword.