The number of days listed on your visa is the visa validity: this is the period during which you can use the visa to enter the US. How long you can stay in the US is determined by Customs and Border Protection at the border based on "the period necessary to carry out your business activities." If you're not sure how long this period is (normally, check the stamp in your passport), you can check your I-94 online. You must leave the US (or apply for an extension well before this date) before your status runs out. If you're admitted to the US for six months, then you're permitted to stay for six months.
That said, long stays can invite more suspicion. You can only use a business visa for certain purposes, but you cannot use one to live or work in the United States. Since many business visitors come for relatively short conferences or meetings, you may face more questions to assess whether you're really only going to be doing what you're allowed to do. Repeated long trips can raise even more suspicion.
If you do have a permitted business reason to stay more than 90 days, it's a good idea to have a clear explanation and supporting documents (indeed, it's a good idea even if you're staying for 12 hours). For example, proof you're attending a four month training program or that you're a service engineer repairing industrial equipment on a job that will take five months (within the bounds of what is permitted for foreign service engineers to do).