As much as possible.The more you keep it the better.But after a few hours the expansion should be gone.You can either stop there and take the cock ring off ( a few hours with the cock ring on should be pretty good ) or you can do a quick 5 min jelqing/pumping/ssj/etc to re-gain the expansion,and after that you put the cock ring back on.Again,the longer you keep the expansion the quicker you will gain.
Just make sure it's not too tight because it will cut circulation while you're sleeping.You don't want to wake up with a blue dick
I would never use an [words=http://www.phallosan.com/shop/catalog/default.php?z=eNortjIxtVKyL0pNszWxMFcrSSxKTy2JL0hMT7U1UisoykyxtbBQSy4tLsnPjS8uKcrMS7dVsgZcMMpbEbo%2C]ADS[/words] after a girth workout because your penis is very full and expanded after the girth workout and you cannot get a good stretch.
I would use an [words=http://www.phallosan.com/shop/catalog/default.php?z=eNortjIxtVKyL0pNszWxMFcrSSxKTy2JL0hMT7U1UisoykyxtbBQSy4tLsnPjS8uKcrMS7dVsgZcMMpbEbo%2C]ADS[/words] only after a length workout .