I have no crystal ball but I have Android 15 on FP3 from
This is a community build so it will not be available through eosinstaller … and links like the above are no longer linked into documentation and install page, Install /e/OS on a Fairphone FP3/3+ - “FP3”.
a15 should be good for a couple of years (probably), there may be hurdles and maybe difficulties with a16 in due course, idk.
However FP3 community will install by script just as said in the above install page.