By now, Debian ships quite a lot of Haskell packages (~600). Because of GHC's ABI volatility, whenever we upload a new version of a library, we have to rebuild all libraries that depend on that. In particular, if we upload a new version of the compiler itself, we have to rebuild all Haskell library packages. So we have to rebuild stuff a lot. Luckily, Debian has a decent autobuilding setup so that I just need to tell it what to rebuild, and the rest happens automatically (including figuring out the actual order to build things).
During the last four days a complete rebuild was happening, due to the upload of GHC 7.6.3. During these 2 days and 18 hours building 537 packages took 48 hours of build time and produced 15kg of CO2. That is 94% of all uploads and 91% the total build time. The numbers are lower for the whole of last year: 52% of uploads, 31% of build time and 57kg of CO2. (The CO2 numbers are very rough estimates.)
Note that amd64 is a bit special, as most packages are uploaded on this architecture by the developers, so no automatic builds are happening. On other architectures have, every upload of a (arch:any) package is built, so the share of Haskell packages will be lower. Unfortunately, at the moment the database does not provide me with a table across all architectures (and I was too lazy to make it configurable yet).