I remember having seen a formula of the form
$C=\frac{B}{L}$
that relates a duration $L$(s) of a window to its width $C$(Hz) in the spectrum (defined at -3dB or so). The constant $B$ would be tabulated in function of different windows.
Does anyone know how this formula is called or where I can find more information about it? I searched on Google (Scholar) but dit not find it.