I doubt very much you're getting full benefit from your unit. I also suspect it's providing some benefit compared to water that hasn't gone through it.

Your pre and post-filters are both shot. Probably have been for the last two years at least, depending on your inbound water quality and volume of usage. RO membrane may or may not be OK. Check the the exclusion ratio with a TDS meter....or let Culligan do it for you. (Takes seconds with a little hand-held gizmo. Don't let them charge you for it.)

Also suspect your holding tank needs attention. That's the source of your low pressure. Likely needs some air.

Most of the replacement in RO units is to prevent flow problems.
The RO filters and membranes simply clog up and the volume delivered decreases.
It is very often still nice and clean, just not as much.

If the membranes developed a hole you would likely not have a decreased flow, but now the water would not be filtered.

The state of your RO over time is dependent upon the water you feed it. Is it receiving raw water? Well or city? Do you have a softener upstream? The answer to your question will vary based on your response to those questions. An RO membrane can last for 10 years or be damaged in a few months. Your water quality entering your RO will at least allow me an educated guess, but it will still only be a guess.

The pre and post filters, however, are an entirely different issue and replacement is NOT simply to prevent flow problems. Pre-filters should be replaced at least annually for two reasons. 1) the carbon prefilter will be spent in that period of time, particularly if you are on city water and should be changed to prevent damage to your RO membrane. Additionally, the prefilter is subject to bacterial growth if left too long. The RO filter will remove bacteria, UNLESS it has been damaged. If the RO has been damaged, then the post-filter will be contaminated as well.

The ONLY way to know if your RO is still functioning properly is with testing. You can purchase a hand-held TDS meter cheaply. Test before and after RO - TDS should be decreased by 90-99% after the RO.

If you don't intend to test, you should replace annually. Guessing is not smart. Assuming your water is "still nice and clean" is not smart.

In a nutshell: Replace pre and post filters annually regardless. Either test your water at least annually or replace your RO filter annually. Testing is cheaper and easier.

How do you test it? From the company you purchased the unit?