AWe need to understand first that in the web industry, the sites which are on the first page of Google (top 10 results) receive the most traffic (over 90% of all traffic). Because of this every web-master which achieves top 10 results will what to keep them for a long period of time.
Once one of our customers ends the work with us after a monthly service (let say a 6 months contract) its ranks will stay in Top 10 for a couple of months or even more (this depends on the keywords and site area competitiveness). However, without any monthly work the ranks will start slowly to decrease so we recommend building links on a monthly basis in order to achieve and maintain top ranks results.