Beethoven: 9 symphonies... Brahms: 4 symphonies... Sibelius: 7 symphonies... Tchaikovsky: 6 symphonies. That's 26 altogether. I've never recorded an orchestra; based on session musicians I've worked with, I would expect the recording sessions to last longer than the compositions in question, and I was estimating at an hour per symphony in the first place, giving the figure of about a week's worth of recording time (~40 hours).
But as I said, I've never recorded an orchestra before; my guesstimates may well be skewed. Even so, at the numbers you are quoting, that's quite a bit lower than I would have guessed earlier today. :-)