To answer this question you would need to know what networking speeds that your cabling system can handle. For example, a 40G Ethernet transceiver will require an optical loss budget no greater than 1.9dB. If the current fiber optic structured cabling system has a loss budget over that number it will need to be replaced.
This question often drives another question. What is the optical loss in my current system?
The best way to get this answer is to contact your supplier and manufacturer and find out the specifications of the installed products. You should also learn the basics of loss budget calculation, you can learn quickly in this informational video!
or... How long will it stand up to the daily rigors of frenzied data center activities?
The answer to this question boils down to the components and process used to build the cabling system. It should be no surprise that cheap components do not last as long as quality (more expensive) ones.
Be sure to evaluate your system and keep track of what has broken and/or caused quality issues. A good quality cabling system should be able to withstand the daily rigors and outlast issue #1!
Buyer beware that cheap components are cost savings to the manufacturer and there is no easy way to tell if it is a quality component or not by just looking at it. The key takes away is to question your supplier/manufacturer about the components used and do your homework!
This is a tough number to pinpoint considering the myriad of variables, but you should have a rough idea of what your last system cost was. Also, consider the time and man-hours for the installation! With that number in mind, you should evaluate step 1 and 2 against this number.
Is it better to invest in a system that will last longer and not have to be replaced or save money (cost) on the project?
I would argue that in most cases it is better to have a quality system that will last for generations of hardware! Respect Layer One!