My first class, Advertising 101 (not a joke; literally ADV 101), was held in an 300 student auditorium. What seemed to be a mile away, the gray-haired professor stood at the podium and exalted, "You have choosen a great field. When the economy is bad, businesses must advertise to draw customers. When the economy is good, businesses must advertise to beat the competition." What he didn't extol was, "Get used to unemployment."
Marketing and Advertising is a very unstable career. When the economy is bad, companies cut their marketing and advertising budgets. When the economy is good, companies may bulk up on staffing for a temporary ride. In the 21st Century, lean operations is the norm. I read in "48 Days to the Work You Love", by Dan Miller, the average job lasts 3.2 years. Advertising Age indicates the average CMO position lasts 18 months.
Don't be discourageed, by these stats. Marketing and Advertising can be a fun career, if you like a fast-paced, changing environments. During one of my unemployment stints, I received company sponsored out-placement services. This service teaches you how to "transfer" your skill sets. I didn't want to "transfer" my skill sets. I enjoyed my industry. Working at several companies can really broaden your skills, as well as provide steady growth. The key to survival is tolerance and tenacity. Your company turnover should not be too high, unless you're really unfortunate to pick companies with short life spans.