I seem to recall a rule of thumb of around 2200uF per amp in a full wave bridge rectifier setup.

As for tolerance causing a failure?

Since a capacitor resists change to voltage, the greater the change the more is has to resist. In a real capacitor this causes heating due to the ESR (equivalent series resistance). Electrolytic caps usually have a ripple current rating and an expected life in hours at a particular ambient temp and ripple current. There are usually derating curves in respect to ripple current and temperature so you can estimate the expected life in a given circumstance.

Nippon Chemi-Con have some reading on this:

http://www.chemi-con.com/u7002/toc_ms.php

With switching power supplies the electros tend to wear out causing a failure - this is not due to the capcitor tolerance, but due to the electrolyte drying up- the ESR rises thus causing more heat build up thus making the electrolyte dry up quicker. Eventually the capacitor fails usually by rupturing the can. If you measure the capacitance, it does change much but the ESR does. Therefore measuring the ESR of a capacitor is a better indication of its performance.