I tend to hoard for long periods of time because (a) most of my travel is for business, and (b) I'll buy tickets for leisure travel if they are relatively cheap. I generally try to achieve at least 1.5 cents per mile on my reward tickets, which means that I'll use miles for a domestic ticket (25,000) only if the ticket price is $375 or higher. Most of the time, even for transcons, I can find tickets cheaper than that. Of course, sometimes when the fare is very high, there is no availability for FF reward tickets.
However, when I do use up my FF miles, I tend to do so in clumps of 100K+ because I'll get tickets for the whole family for a vacation trip. I recall back in the late 1980s or early 1990s, when Continental changed its reward mileage thresholds (from 20K to 25K for a domestic ticket), my wife and I cashed out almost all our miles. I think I'm back over 200K for Continental and close to 500K for American, with smaller amounts in US Air, Delta and United. Time to cash in again!
