Quite honestly, it’s down to personal preference. I’d say the generally accepted timescale is once a week, but if you leave it longer it won’t make you ill unless you suffer from asthma or other respiratory problems.
The longer the sheets are left, the denser the dust mite numbers, which means higher quantities of their faeces, which is what can trigger an asthma attack.
Also, to keep dust mites down, don’t make the bed as soon as you get up, because if you do they’ll carry on breeding in the warm and humid atmosphere.
Much better to pull back the covers and open a window to lower the temperature – that way you’ll reduce the population.