This depends on the type of toothbrush you purchase. All of them should last at least 3-4 months. If you’ve purchased a toothbrush without replaceable heads, they’re meant to last this long and then be disposed of, and a new replacement toothbrush purchased. This is why they are often inexpensive. If you purchase a brush with replaceable heads, then the brush can last for years as long as you take good care of it, regularly replace the brush heads and batteries, and keep it clean and dry.