They're already trying to phase it out PS3 by giving you an opportunity to upgrade to PS4 version for free before the end of the year. It is likely that we may see a phase out about 6 months into the first expansion if the PS4 numbers start drastically taking over the PS3. I say 6 months as we don't know if the first expansion would auto upgrade along with the base game to the PS4. If the upgrade system applied to expansions, I could see it in 3 months depending on PS4 numbers then.

If the PS3 numbers were less than 10% of the user total, I could see a justification to phase it out with ample warning (3 months in advance at the very minimum) and an incentive to upgrade like transferring licences of the base game and expansion to either platform. Until it is a very, VERY small minority, it just wouldn't be a smart move to throw away a huge chunk of change by phasing out subscribing PS3 players.