Alison: Yes, they will need replacing at some point. You'll just find that the fabric loops, which have the exfoliating properties, will snap at some point.
Basically, if you feel that they are still exfoliating and working really well, then you can carry on using them. Just wash them every now and then (maybe a little scrub a couple of times a month) in something like ABC Skinwash or an antiseptic that you already have at home, like mild Savlon, just to make sure that no bacteria is building up on them over time. Rinse them thoroughly afterwards.