No, I used "15%" merely as an example. What I mean is that if you are 10% (for example) and you decide to stop bulking when you reach 15%, this can happen in 3 months but it can also happen in 2 weeks. In the second case, you will have obviously gained a lot of fat.
For example, if you are 80kg and 10%, then your lean mass is 72kg and your fat mass is 8kg.
You can be 15% like this:
100kg (85kg lean mass, 15kg fat)
...but this could also happen:
88kg (75kg lean mass, 13kg fat)
In both cases you are around 15%, but in the second case you have gained 3kg muscle and 5kg fat.