Sometimes, for instance when there is a sticker on top of the text that says whether it is a DCC90, DCC75 or DCC60, you have to guess the tape length.
Not anymore. I have created this diagram that show you just how long the tape is by looking at the casing (backside):
It is really silly, some brands (hello BASF!) mention the length of the tape only once on the casing, and that is exactly where normally a title sticker would be placed. By looking at the diagram, you will no longer have to guess. The black circles in the drawing are holes in the casing.
DCC90 would mean 45 minutes on side A, and 45 minutes on side B, totalling 90 minutes. In practice, there will always be a few extra minutes of tape in the cassette. This goes for all the formats.
DCC75 is a bit strange, it was also not used in normal analog compact cassettes, but it would gave you 2 times 37 1/2 minutes. Not quite sure what the reasoning behind that was.
DCC60 is 2 times 30 minutes.
Have fun with it!