If you have 7-day pass (which I'm currently travelling with myself) it means I have 7 travel days that can be used within 30 days, counting from the first travel day. So for example: if during 1 day, I travel a long distance, with maybe 3 different connecting trains, this is still 1 travel-day (as long as the last leg of the journey starts before midnight). But let’s say that another day I only travel a short distance, maybe a 2-hour train ride. Then this will also count as 1 travel day. So it is good to plan your travel days, and sometimes It might be better to just purchase a short train ride as a regular ticket, instead of using up one of your 7 travel days.

Note it lasts for calendar months not a specific number of days. So if you start on Sep 2nd with a one month pass all travel must be completed by 2359 CET on October 1st. (30 days). However if you were to start Feb 2nd then you would need to complete by 2359 Mar 1st (28/29 days).