Rugmaker,
The soils that surround the fields make a big difference, so the fields in some soils may drain quickly but not in others. Clay is notorious for not draining well and having a short life expectancy while fields in loam will drain better and the fields last longer. So it is difficult to give a general life expectancy to someones fields. Comparing with neighbors is usually helpful. Water (rain) flowing arcoss the fields can also reduce their ability to drain. (We are on a hill and have a swale to divert most of the run off away from our fields.)
The problem with bleach is that it is anti-septic...kills the bacteria that digest many of the solids. Not all solids break down well especially things like toilet tissue, soaps, and grease. The outlet from the tank to the fields is set to be low enough for the top layer of liquid to bleed off and high enough that solids will not move into the distribution box and then into the fields (and clog them.) If the solids fill the tank above the outlet, or the baffle collapses, or so much water rushes into the tank that turbulence stirs the solids up to the top, some of those solids can flow into the fields. Levels in an unused tank will fall below the outlet but can go no lower. The tank is pumped to keep the level of solids low.
Sometimes, a cement tank will collapse (usually from salts), or a distribution box become clogged, but most system failures are in the fields. The life expectancy of fields with average care in our area is 20 years. In better soils and given better care, they can last at least 50 years.
If toilets are draining slow, at least have them checked. Good luck.