You are not waiting till the missile gets to 25km before you launch or fire at any incoming threat, that 25km is where you would like to hit your target and depending on height speed etc it could be further out... (again we don't know the exact range of the SeaCeptor and for good reason)
However fire enough missiles at any ship it an over whelm the CMS and missile guides of any ship.
I agree that the vessel would likely fire before the inbounds were 25 km away, but as they approached the umbrella coverage. Going off the publicly available data (yes, I am aware that the reliability of such data can be questionable) it should take a Sea Ceptor missile ~24 sec to reach the 25 km range mark. Depending on the inbound and it's flight profile, an inbound might not be that much further away before a Sea Ceptor would get to that outer range mark. Using an inbound YJ-83 on a cruising speed approach of ~0.9 Mach, that could get fired up whilst ~32 to 33 km away from the targeted ship if the Sea Ceptor is getting fired for max range. The ship could very well be aware of an approaching in bound for multiple minutes, but just not have options available to respond until the inbounds get closer.
Where this could get problematic is if there is a need for a ship to re-engage an inbound that became a leaker, or if there is a saturation attack which seeks to overwhelm the CMS and missile guidance. Not sure what the capabilities of the Sea Ceptor and CMS 330 are in terms of handling and guiding multiple outbound SAM's against multiple inbound threats. From published data on the RAN
ANZAC-class frigates, the CEAMOUNT illuminator for the ESSM Block I can engage 10 targets simultaneously. I would imagine that the ESSM Block II which has an additional seeker and extra guidance capabilities, there could be both a larger number of targets engaged at once as well as greater range to engage inbounds as they approach.
Where I grow concerned is that it seems people might grow complacent, thinking that because a ship holds/can hold
nnn-number of Sea Ceptor missiles the vessel and nearby assets are protected, when it is distinctly possible that the ships sensors, CMS and missiles cannot effectively engage numerous inbounds even though there are plenty of missiles onboard.
Now of course I could also be quite wrong and the CMS configuration along with Sea Ceptor guidance could permit a significant portion of the VLS to be emptied all at once, but that does sound a little unlikely.