How long is the suppressive efficiency by siRNA or shRNA - (Nov/02/2012 )

I am considering of choosing siRNA or shRNA way for suppression. But I am not sure whether shRNA can be really stable and how long the inhibition of siRNA can retain.

siRNA transfection has transient knockdown lasting for 5-7 days. Stable expression of shRNA should give persistent knockdown as long as the shRNA is expressed.

pcrman on Fri Nov 2 16:45:16 2012 said:

if the shRNA integrates into genome, then should be stable knockdown. But if not, whether the shRNA effect will be similar as siRNA to be temporary (few days) ? or the temporary knockdown of shRNA can also retain longer than siRNA?