That depends on what usage you have on other parts of the filesystem. If you’re sure nothing does periodic writes to the internal memory, it should last pretty long. But as soon as your ssd will fail and unmount (and it will happen), the router will happily provide the /srv folder on the internal memory and then the decay can be pretty fast.
My suggestion is: make sure you won’t lose anything irreplaceable when the internal memory dies, have a serial cable ready, and use the internal memory as long as it lives. By moving your system to SSD, you’ll lose the physical-button rollback feature, and that was something I really miss now.
If you want to be on a bit safer side, I suggest adding a script to cron that would periodically check if srv is on the mounted drive, and if not, remount it readonly and shout loudly on you.