Many businesses are like that.  For a car salesman, when will the next customer walk in the door?

On the OP's question, it's a good feature, but the question is, how would we word it so people don't expect to get something after that specific amount of time?  "it said 7 days and I waited more than a week and didn't get anything!"  Approx, average, but still they're going to think that way.  It can't be a whole sentence, unless we think of somewhere else to put it, but where would that be?  Suggestions?

The difficulty quadrupled a few minutes ago to 181.54.  It's going to take typically about a week to generate now.