No experience with the guardians myself but some thoughts that might be helpful:
I've had an "industrial" PH probe sitting in a relatively low-ec solution (circulating) for around six months and haven't had any issues, stable PH readings. These particular probes are labeled for continuous solution contact.
For instance, https://www.atlas-scientific.com/_files/_datasheets/_probe/IpH_probe.pdf claims a life expectancy of 4 years (1 year cal) fully submersed in a salt solution.
For the Guardian PH probes, they state https://www.bluelab.com/Product-Manuals/Guardian-Monitor-Manuals/Bluelab-Guardian-Manual-ENG-JUL13.aspx :
The life time of a pH probe depends on the environment it is used in and the way that it is treated. To receive a long life from your pH probe, please ensure you follow the guide below.
They do not give an explicit estimation because it's a function of the type of solution and how well the probe is maintained but would assume they have a relatively similar service life as other "industrial" type probes. They give a six month warrantee and a replacement probe has an OEM price of ~50USD.
But, as 99 says, the probes will eventually degrade. Clean, re-condition (in KCL) and re-calibrate them every so often (monthly for instance) to extend the service lifetime.
The primary mechanisms of eventual failure will probably be 1) fouling of the glass bulb with biological films, iron, or calcium 2) fouling of the reference junction 3) depletion of the electrolyte 4) biological growth in the glass bulb. Regular maintenance will extend the life by addressing these problems.
For the most part, PH probes will have a service lifetime and even the best will eventually fail or slow. The maintenance part play a large role on the duration of the service lifetime.