Well of course it depends on the factory in question, but here's some rough eyeballing it figures. Though rather than tons-per-year or Mechs-per-year, maybe cost should be an issue? Regardless, here's my estimates.
Tech level C: Hand built by top scientists in experimental laboratory-factories only, 300 tons per year max
Tech Level D: Succession wars/Age of War factory, 3000 tons per year maximum (30 Mackies or 150 Wasps)
Tech Level E: Star league or post 3050 Inner Sphere facility, 6000 tons per year (60 Atlases or 300 Locusts)
Tech level F: Clantech or Comstar/WoB, 9000 tons per year, (90 King crabs or 450 Mercuries)
It might be advisable to build the largest and most expensive single component(Fusion engine) in a separate facility, especially if you're using it for multiple designs or if you're using XL engines which have quality control issues in the fluff.