Just how big of a problem is this? Well, no large studies have been done to date, but one smaller study showed clearly this is a serious issue. Researchers looked at men with erectile problems that had taken Viagra successfully and continued use over an extended period of time. Of these men, fully 37% had to increase their dosage by 50 mg! In a second cohort 50% of the Viagra users discontinued due to the decreased effectiveness of the drug. 
CAUTION: Do not quit any medication without consulting your doctor. Viagra and Cialis can lower blood pressure slig
This also creates a path to dependency as well. Imagine this all-too-common scenario: you start on Viagra or Cialis and notice some slight side effects, maybe mild ringing in the ears, visual issues or stomach upset. You are so happy to get your erections back that you ignore any negative aspects. However, as the months roll on, the nagging doubts increase: “What is this ringing in my ears doing to my hearing long term?” Or maybe the stomach upset or headache just gets worse over time to the pont where “the cure is worse than the disease.”
What is the solution? Again, consult with your doctor, but read my link on Eliminating Cialis and Viagra Dependency for practical suggestions to discuss with him or her.
1) Curr Vasc Pharmacol, 2006 Apr, 4(2):89-93, “The management of phosphodiesterase-5 (PDE5) inhibitor failure”
2) The Journal of Urology, Sep 2001, 166(3):927-931, “LONG-TERM EFFICACY OF SILDENAFIL AND TACHYPHYLAXIS EFFECT”