This depends on the company's goals and needs. For most local businesses that have done little-to-no SEO in the past, a 6-month campaign to implement all the needed changes at around $500/month is usually enough of an upfront investment to see results for years afterward. It’s also possible to do it for less if businesses want to move slower as they tackle each step.
Don’t be taken by directory companies that claim to just list you around the web; you need a full campaign to make a difference. Directory listings are only one piece of the SEO pie.