With a voltage stabalizer the bulbs last halfway to forever.
That's not the function of a voltage stabilizer, it only assures a more constant light output (does not compensates for lamp aging). OTOH, a transient suppressor (much cheaper) will do the trick. To be fair, some stabilizers also include a suppressor.

The other trick is to set the voltage on back of the column to 220 and run the enlarger on 110.
. This way your lamp will last forever... but you will get less than a quarter of its light output and with a severe color shift towards orange. Not viable for color work, and with unpredictable results with multicontrast papers (a softer contrast may be expected, and harder grades may be not reachable).

Finally, there are stabilizers that include a "ramp output" feature. This will, in fact, extend the lamp life many times (forever?).
The Leitz one does not seem to be one of those, but that's just a guess.