FFE is a safe starter for Protoss.. due to the wall off there's not a whole lot of options for early harass.. however these is great options for map control, you can move Overlords to his 3rd and 4th and start dropping creep, you can take your 3rd and drone up hard.. thing is you gotta scout and see what he's doing. If he makes robo after his cybernetics core is done, expect an all-in.. if he makes Twilight expect either Blink Stalker all-in or Dark Templar from Dark Shrine.. if he makes 2x stargates expect phoenix -> voidrays with possibly an oracle or two to try to snipe your workers
Anyway basically the most common thing Toss tend to do after FFE at our level of play (Gold League) is the Immortal Sentry all in. Things to look out for, what time do they take their gas at the natural.. if they have both gasses down on their natural prior to or just around 7:00 expect the All-In push.. also you need to fly an overseer or sacrifice an overlord to peek into their base it is so vital.. because the first attack can actually come as early as 7:00 if they decide to pressure you with zealots and a small number of stalkers..
Usually though they'll be doing the standard Immortal Sentry which means they'll get robo as soon as cybernetics core is done, they'll get one zealot to complete the wall-off then they'll pump sentries. After the robo is started they'll make like 4 or 5 more Warp Gates.. they'll get like 8-12 sentries, 3 Immortals and an observer from the robo, and they'll fill the rest up either with Zealots, Stalkers, or both.
This all-in pushes out between 9:00 and 10:30 (10:30 being the absolute latest). However it's not common for any kind of pressure or even for them to scout when they're going this route.. so they are very vulnerable to the Swarm Host all-in, provided you land the Nydus outside their base no latter than 8:30 usually all they'll have is a few sentries and their first Immortal or two which can NOT defeat the two swarmhosts / 5 queens at all.
However one thing to look out for when doing the swarm host rush/contain is that they may break down rocks and exit their natural, go around your swarm hosts, and then just enter into a base trade.. they could also escape out the same path and take their 3rd while you're killing your natural.. so its important not to let it become a base trade IF they do that, you can always retreat your swarm hosts and queens back to your base through the nydus and try to hold your base.
Now if they are doing the OTHER two-base all-in.. the one with collosus... what they'll do is send Zealots out occasionally and force you to make zerglings and/or roaches.. this will cut your drone count behind and they don't really intend to kill you with this (but if you're not defended you WILL lose your 3rd or natural) but they really just want to delay your tech, delay your worker production, and buy them time until they can get 2 or 3 colossus out with maybe 2 or so Immortals maybe 6 or so sentries and a bunch of stalkers. That will usually attack anywhere from 11:00 to 12:30... it's considerably harder to hold off, but it's also much riskier for them because it gives you time to get mutas which will shut it down.. IF you are able to hold their first couple zealot pokes.. if the zealot pokes do their damage you'll be behind getting mutas out and probably die.
So the swarm host all-in is really more of a coin toss. I'd say you have about 50/50 chance of killing them then and there depending on which build they're going they'll either attack you before you are ready, or they won't.
It's much more safe to just play macro ... get your 3rd asap when you see them do FFE, poke in and get vision around 7:30 keep your 1st overlord where it can see their gas timings.. try to spot if they're going stargate or robo or freakin dark shrine or what.
You need to get roach warren up pretty fast when they do FFE imo, you can cut some workers to get like 6 or so Roaches for any early zealot pressure, ofc you need ling speed as well. Many suggest getting baneling nest too but I almost never use banelings in ZvP (maybe that's why I'm stuck in gold >_>)
You want to get an evo or two and at least get carapace armor +1 before any kind of timing ie you want it DONE and starting your next upgrade I'd say no later than 8:00 which means build it around the 6:30 or 7:00 mark.
From here it deviates depending on what's your style.. some say get a hydalisk den make some Roach/Hydra fill in excess minerals with lings when you're out of gas, poke at their wall but hit and run dont' over commit don't get trapped by forcefields basically just harass just to annoy and delay them, macro up behind it and make ready for a tech switch to hydras. Nasreth taught me that one... start out the early/mid game with Roach Ling Hydra or just Roach Ling, but you basically get your spire soon after your first group of upgrades/roaches are done, and then just try to fully saturate (cont