Most likely you have access to statistics reports that show you some valuable data about your visitors.  (If you don’t I would definitely as your web developer or web host about this.  They should be able to setup Google Analytics for you, which is a very powerful tool available for free from Google.)  In addition your ecommerce software should have reports that can assist you in better evaluating your online return on investment.

Here are some key metrics you should pay attention to:

• Monthly Unique Visitors
• Monthly New Customers
• Average Order Size

One metric that is commonly looked at is Average Time Spent on Site.  While this is interesting there are many problems with using Time Spent on Site as a judge of your websites ability to engage visitors.  You can read more about this here Understanding the "Time Spent on the Site" Metrics