In general an x-ray tube is just an incandescent light bulb. It’s filament has a short lifetime. No amount of tube maintenance will prevent that.

Filaments can withstand ~20,000 (Toshiba Tubes) to ~60,000 (Varian Tubes) exposure cycles (with a +/-10,000 variance).

Data above based on internal testing performed at Summit Industries on Toshiba 7239 tubes and Varian RAD8 tubes.