On average like two days for one place. I did interrail for a month, Berlin -> Stockholm -> Gothenburg -> Copenhagen -> Hamburg -> Amsterdam -> the Hague -> Antwerp (2h only) -> Bruge -> Brussels.
I had like 2/3 days and it was a very nice experience but in the end all the traveling was very exhausting. The distances seem also quite far. Belgian beer and food should not be underestimated tho