Don't worry Inna. You'll know your heads are worn if, after cleaning and demagnetizing, and worse case, re-aligning the deck, it no longer sounds as good as the source material. I firmly believe that the rate of head wear is largely dependent on the quality of tape used.
I'd agree with Marak that the heads in bottom of the line decks of any make probably don't last more than a few years. I disagree that using higher quality metal tapes, like TDK MA or Maxell MX, accelerate head wear. If anything they prolong it. Using cheap tape requires more frequent cleaning of the heads. Good type IV (metal tape) is more uniform than its lesser cousins. It doesn't shed its particles as readily, and it is the grit of loose particles that contribute to head wear.
Nak addressed head wear by using dual capstan transports in some of their pricier decks. The "Asymmetrical Dual-Capstan Transport" contains a lifter to move the pressure pad inside the tape out of the way, so it no longer pinches the tape to the head. This reduces scrape flutter and prolongs head wear. I've had my 3-head Nak CR-3A since new in 1988 and it still works and sounds as good as new - using metal tape exclusively to archive hundreds of record albums. So if yours still sounds good, all I'd recommend is regular cleaning, and regular exercise.