Data packs you buy for use in Switzerland always roll over and get added to your data credit. Any data you don't use from a data pack you bought gets added to the next month.
But roaming data packs are only valid for a month. In other words, roaming data does not roll over to the next month. What do we mean by a month? A month is the number of calendar days in the month you bought the data pack.
So if you bought the roaming data pack in September, you'd have 30 days to use the data from the date you bought it.