Equipment (Time to Replace?)
You know that feeling the very first time you use a new toothbrush? Your teeth feel so much cleaner than the last time you brushed with that old toothbrush, and if you're like me you probably have a second where you think to yourself "I really should have gotten a new toothbrush sooner". Where am I going with this tip you might ask? Well, this is the best analogy I can make for getting a new head for your curling brush. You look at it and think, "That's still in pretty good shape", or "It's not too dirty". In theory you might be right, but if you've gotten to the point where you aren't sure, then it's probably time for a new one.
If you are going to play a round of golf you probably aren't going to use old scuffed up or cut golf balls, you're going to dig through your bag to find the best balls you have or possibly buy a new sleeve. In short, if you are going to do all of the hard work of sweeping up and down the sheet for two hours, you might want to get a little something, you know, for the effort ("Caddyshack" reference).
The guys you see sweeping on TV are probably switching heads every couple of games. This, of course, is unrealistic for anyone who isn't sponsored by a brush company. My general recommendation is if you play once a week you probably need one brush head per season, twice per week two heads per season, etc... This will vary slightly if you have really clean or dirty ice, or if you are always skipping and don't do much sweeping, but you get the point.
The long and short of it is, if your brush head starts to feel smooth, it's not very effective any longer. Grab a new head and put it on your brush, sweep a rock, then put the old one on and do the same, you'll quickly understand the toothbrush analogy. To finish, I have a tip that will help your heads last longer so you don't think I'm just trying to peddle merchandise here. When you are practicing put on an old head, and save the good heads for games. Cleaning stones in and around the hack make them very dirty very fast, and you're less likely to clean them if you're just practicing. This will kill a perfectly good head pretty quickly, so use one that's already shot.