In the old days (at least for me) a receiver in the HA body lasted a long time, four, five years before needing replacement. Moisture was usually the cause for replacing a receiver along with the receiver just wearing out over time. Then about ten years ago a lot of aids went to receivers in the ear mold. Any way to address your question, due to my hearing loss I’ve always felt it difficult to distinguish when a receiver needs replacement. In most cases the receiver does not completely shut off. I’ve been told a receiver in the ear mold can last anywhere from a year to three years - with the average being replaced after two years. I wish there was some warning system built into HA that alerts the user when a receiver starts to fail. A muffled sound could be a sign the receiver needs replacement, along with sound distortion as another warning sign. Personally I’d rather let an audi do a receiver swap in his/her office, but some HA’s force you to mail you aid for new receiver replacement. At as anything with HA’s a small simple receiver that should cost $30 - $40 could easily cost $110 or more. If you wear two hearing aids probably best to have both replaced at the same time.
But before you blame the muffled sound on a defective receiver, check your aid for ear wax build up, especially if its vented. Ear wax buildup can cause a hearing aid to offer a muffled sound, especially so if the air vent to plugged up.