Answer:
300-465 mb
If the Questioner means the "media storage" (how much data can a tape cartridge store) the answer is different.
Just like the old cassette & VHS, there are different storage sizes depending on the tape's label. We use LTO-3's cartridges that can hold up to 400GB of data (800GB if compressed). Current mag-tape storage can range from 100GB for the old LTO-1's up to 1500GB (that's 1.5TB!) for the new (and wildly expensive) LTO-5's.