When does a Window function's sidelobe level become out of band attenuation? Another question I'm struggling to answer by myself. 

For a Window function, say Blackman (response pictured), what is the out of band attenuation? If I'm designing a filter with out of band attenuation of 40dB, is it the level of the first sidelobe that I'm interested in? Or is the out of band attenuation dictated by something else (like 3rd sidelobe level for example?)

Thanks!
Windows with low sidelobe levels have large transition
bandwidth's. Transition bandwidth is inversely proportional to N for a
given window.  Indeed, the ratio of transition width over sidelobe level
in dB for fixed length N is virtually a constant. 

We can use the following empirical formula to estimate the required filter length
   

where,


N is filter length,
A is the out of band attenuation in dB
fs is the sampling frequency