The data provided by the ESA INTEGRAL mission have revealed the presence of a huge new population of HMXB sources. Among them, an important fraction belong to the fast-growing class of Supergiant Fast X-ray Transients (SFXT). We present a summary of our study to characterize the outburst frequency of SFXT using INTEGRAL data corresponding to our on-going long-term monitoring campaign. Assuming a clumpy wind accretion scenario, the differences between SFXT and classical supergiants X-ray emission, can be explained by simple geometrical differences in their orbital configurations.