How long does it take for a brain aneurysm to develop?

Unknown: There is no good answer to this question because we have only limited data on exactly how, when and why brain aneurysms develop. Once an aneurysm is identified, if it is non-ruptured and small (<5mm in size), we often follow these with yearly tests to see if the aneurysm is enlarging. In these cases, they tend to grow at varying rates or not grow in size at all.