When Ma Bell owned the whole US telephone network, the reliability of dial tone (that is, picking up the telephone and hearing a dial tone) was one minute down per year. I worked for a company that sold equipment that achieved around twenty minutes down per year, on average. We had a Japanese customer that bought an extra system so that there would be no downtime during planned upgrades.
Fifteen minutes a month is miserable. It's bush league.
I guess in a world where software products measure their downtime in hours instead of seconds we forget what solid engineering can accomplish.