As with anything in life, the more players there are the greater the competition. If more websites are advertising the same product as you, ranking yours will be more difficult. If you are in a very specific sector with limited competitors, rank change will come easier.
For instance, if you are a restaurant owner in downtown Manhattan ranking your site will take much longer. That is compared to that same restaurant owner in a smaller town far from city limits.
There are also factors regarding the competition that should be investigated. First, are other similar businesses online? If so, what do their websites look like? Do they look professional and easy to navigate or outdated? Because it is a powerful tool, SEO experts research competition extensively.
Second, the keyword difficulty level is a major component. If your target keywords are broader, the more time it will take to rank. If your keywords are more specific, then the less time your site will take to rank.
The competition is a large factor in how long it will take to see SEO results. A more popular product, with a broad keyword, will take longer because you have so much more competition. However, this is not something to be discouraged by. Because the potential customer base is larger, once your site is ranked, you will receive a ton of new views daily!