It's a question of markets, I think, and it's something we haven't really decided on. 1 hour covers IM, mostly. 8-24 probably covers most e-mail use (maybe?) What we don't want this to become is something like Rapidshare, and limiting file lifetimes is our attempt at not letting that happen. The service is designed primarily to share files with people you know personally, within a finite time period. But we'll see how it gets used, and maybe change our policy.
If you're worried about that you can always expire either when time limit is exceeded (say, 2 weeks) or download limit (downloaded 5 times?). Or you can make download times degrade when the file is downloaded more frequently -- the 100th download will go at 10kb/sec instead of 10mb/sec. There are so many creative solutions that you don't need to put any rules in place now.
Also, I think most of us (myself included) worry about abuse too much. If people start abusing your service you will have the data to deal with it effectively -- you'll think of a solution because you'll have to. And if it turns out people aren't taking advantage of a friendly download policy -- then the service ends up being more useful for honest users.
I really appreciate your insights here, you bring up some really good points. I went ahead and wired in as much data collection / monitoring as I can so through the beta I can see how the service is actually used before I get too concerned with abuse prevention.