That's a really tough call. Are you thinking about replacing the tile floor? Do you have any spare tile if not, since they will likely break some or all of those that will need to be removed?
Where is the flange now? Is it on top of the finished floor (ceramic)? Or, did you end up using a double wax ring to try to get it to seal? If so, you are seeing one of the consequences of not updating the flange to the designed parameters - the flange is supposed to be on top of the finished floor, and anchored through it to the subfloor. This makes it (usually) more evident if it develops a leak.
Plywood isn't very strong unless you provide blocking or you can span at least two, and preferably three joist bays. If you only span the one bay where the pipe comes up, it won't be very strong unless you also install some blocking to hold up the edges of both the new piece you install and the one you cut. It is very possible that in hammering in the new stuff, you could crack some tiles or grout on what was undisturbed.
Removing the old tile from the subfloor is a very chancy situation, and they likely won't come up intact.
It also depends on whether the tile was set in a mudbed, on cement board, or what. If a mudbed, the cold joints if you were to just patch this would be alikely place for the tile to crack, and might suggest replace the entire tiled floor.
I can envision what might need to be done, and see a whole bunch of gotchas. The damage might go much further than you think.
The mechanics of tearing out enough to replace the subflooring isn't too bad. How far they'd need to go, and what you want done after that to either try to restore what is there, or remodel can really add up.
Not sure anyone that can't see it would be willing to venture a guess, sight unseen. I know I don't have enough experience to tell you how long, but I'm sure I could fix it if I had to.