When does a Window function's sidelobe level become out of band attenuation?
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