Well, in my humblest of opinions, a full fledged business plan is not really necessary until you are looking for angel and institutional funding - Although you do not necessarily need a fully detailed business plan for angels, I think it adds a level of professionalism and lets potential investors know that you have thought out your business

If nothing else, putting together a BP is a great exercise for you and your team to go through to minimize risks --

My company has ~15pg BP at the moment