There is not really a set time of when the bed linen should be changed. It is not like there is a law on how often you change your bed linens, most of the time it fully depends on the situation that you are in. The aspect that you have to remember is even though you may replace your linen at night, when you get up in the morning it will probably be quite dirty again. I like to call this "invisible dirt", the reason for saying this is because when you are asleep your body will shed hair follicles and dead skin cells which are hardly noticeable, but are always there. In an ideal world it would be great to have fresh linens on a daily basis, but in this current day and age people not only do not have the time but they do not have the resources.
As a simple rule most bed linen should be changed on a weekly basis, if of course you have a child or a baby whose nappy may leak, or they may spill food or milk in the bed then it would have to be changed more often than usual.
If you are an allergy or hay fever sufferer, than I would recommend changing the bed linen on an every other day basis. The reason I say this is because especially for hay fever sufferers the pollen has the ability to float and land on the bed, and if you are only changing that on a weekly basis, every time you lay out on your bed you will be surrounded by pollen, going into your airways. Therefore effectively leaving you irritated throughout the day.
All in all the only correct answer to give is, change your linens whenever you feel they are getting smelly or dirty. You may only stay in your flat one day a week so therefore you may be able to use your linens for a longer period of time. Whatever you decide just remember that there is no answer set in stone. You must decide this yourself; it has to do with your lifestyle. Finally if you are planning to buy new bed linen and want to know whether to throw your old ones away, you should carefully evaluate whether the old bed linen is still usable if it is you may want to give it to a charity of your choice.
