Most business plans ignore bad debt altogether. I don't think anybody could come up with a standard because it would depend on what business you're in and where, what kind of customer relationships you manage to keep, how well you fulfill the promises you make, the economy and so many other factors.
My advice would be to call up three or four similar businesses, preferably in areas in which they don't compete with you (maybe just look five states away) and ask them. That's hard, of course, because most people wouldn't take your call. But it might help.
Tim Berry is the chairman of Eugene, Ore.-Palo Alto Software, which produces business-planning software. He founded Bplans.com and wrote The Plan-As-You-Go Business Plan, published by Entrepreneur Press. Berry is also a co-founder of HavePresence.com, a leader in a local angel-investment group and a judge of international business-plan competitions.