1. Analyze who you want to target market to be.
2. Analyze if your website is attracting that target market.
3. Review the customer usability of your website.
4. Discover problems with the site that prevent people from converting from simple visitors into costumers.
5. Establish a long term plan with stages to correct the problems you discovered
6. Review results of those steps and continue with the previous plan or improve it.
An SEO company might take 2 to 3 weeks to complete each of those steps. the Analysis phase may take longer if you have a small number of visitors to your site, since there's less accurate data collected.
Take time to evaluate every potential SEO agency. Get to know them and how they will work with you. Don't hire them unless you are fully satisfied because they are as important a choice as hiring your accountant.