Factors Within Your Control
You could create advertising campaigns and check them once a month, however that would not be in your best interest. There are many variables to take into account with your search engine pay per click campaigns (PPC). You obviously have the advertisements themselves to compare against one another and keywords you are testing. These need to be watched closely so you can make the appropriate adjustments. While these variables are within your control, other variables are not.

Factors Beyond Your Control
There are other factors beyond your control that will effect your campaign. Keywords that you are using may have little competition today and more bidders the next day. Others, potentially your competitors, may have found these keywords as well and are increasing the competition. If you are not watching the words your ads may not obtain the amount of traffic they did prior to the competition.

Aside from your competitors you need to watch the trends of your customers, they can change their search habits and search terms as well. Particularly if a new product, service, or feature has hit the market in your industry then your customers may change their search terms which will dry up your flow of visitors. This is yet another category of search engine marketing optimization to be cognizant of.

Search Engine Marketing Optimization
There are multiple factors within your control such as changing keyword bids, targeting variables, the advertisements themselves, your corresponding landing pages and more, however all need to be watched continuously. Consistent search engine marketing optimization of your campaigns will lead to the best possible return on investment.