Hello! Both APA and MLA styles require that numbers with decimals be expressed in numerals, so you would use 5.5. However, business writing does not necessarily subscribe to any one academic writing style. You might think about whether you need to be exact about the amount of time or if you can approximate. For example, it might sound more professional to say "more than five years" or "nearly six years" if you do not have to be precise about those six months. It will depend on what you are referring to, so use your best judgment. If you would like a more specific answer, feel free to reply with the sentence in question and I will be happy to review it for you.