If the Earth suddenly become a thin shell of same radius then length of a day(in hours) becomes

In this case the total angular momentum will be conserved,

${\mathrm{I}}_1{\mathrm{\omega }}_1={\mathrm{I}}_2{\mathrm{\omega }}_2, \mathrm{where} {\mathrm{I}}_1=\frac{2}{5}{\mathrm{MR}}^2, {\mathrm{I}}_2=\frac{2}{3}{\mathrm{MR}}^2$

we get $3{\mathrm{\omega }}_1=5{\mathrm{\omega }}_2$ 

So the time period of one rotation  will 5/3 times 24 hours

So, the new duration of one day =24 x 5/3= 40hours