Experimental creep curves and long-term strength curves for viscoelastic compressible materials have been obtained at room temperature for a wide range of stresses. The residual density change (loosening) of the material is considered as the main damage factor. A modified creep equation for the loosening medium is proposed, which can describe the second and third regions of the creep curve. Based on proposed equation the criterion of long-term strength is formulated. It is shown that in the region of relatively high stresses and under the assumption of incompressibility of the material, this criterion coincides with the known Hoff law of viscous fracture.