I think for most common items (cameras, servers, monitors, switches, etc.), if they don't arrive in under two weeks, it's too long. That being said, customers were usually told 4-6 weeks before we hit their site, so there was some wiggle room.
As far as what to do, I'm not sure. When Sony had issues because of the tsunami (or was it the earthquake?) a few years back, there were a few projects which ended up with Bosch or Panasonic. In cases where we couldn't substitute, we went to any and all sources to find cameras. More than once, we found stock from less common sources, like CDW or Amazon.
You do end up taking a hit on price, usually. Sometimes, explaining the backorder situation to a customer, they're willing to accept an increase in cost to not back up the project. Othertimes, you take a hit it to not be in breach of contract.