How fast do can brain tumors develop serious symptoms?

Highly variable: Some are very slow like meningiomas in the elderly, some very rapid, such as secondary metastatic spread from a primary source. Tumors in children tend to be localized in the mid-line posterior, and can rapidly create havoc due to location. Primary brain gliomas can start off seemingly localized and benign but can transform into horrendous glioblastoma multiforme.

Depends on the type: Brain tumors can develop symptoms very rapidly. This depends on the type and the location. A tumor that is near an eloquent area (ie. Controls motor or speech) can have serious symptoms very rapidly whether it is benign or malignant. This is because it can irritate and invade these areas that then manifest the symptoms. A frankly malignant tumor can have evolving symptoms on the order of days.