Reasonable? Any weight that can be pedaled uphill or carried upstairs is reasonable. If I have to dismount and push the bike (barring deep snow, which is just an unreasonable riding surface), then the weight/gearing is unreasonable. If I can't carry the bike up the stairs to my second story apartment, then the weight is unreasonable. And I imagine that this varies from person to person.

My current ride, with the bare minimum of gear (that which never leaves the bike: fenders, racks, lock and chain, lights, pump, spare tubes and patch kit) weighs in at 48 lbs. It's especially heavy at the moment on account of my winter tires. On top of this I add panniers with 20 lbs of textbooks, homework, and spare clothing. I regularly stop and pick up a 12 pack of bottled beer on the way home from class, and strap that to the rack as well.

The most I've loaded onto the bike was 60+ lbs of groceries and beer, during a particularly large Costco run, and I spent an awful lot of time in the bottom gear range coming home. I made it home just fine though, so I still consider that a reasonable load.