From what I remember, only a few minutes? (Maybe something changed cuz that was a while ago)
We don't really know the exact timeframe, just that he went back to before his fight with Cosmic Garou.
However, he showed absolutely no effort in doing this, so it's unreasonable to say that that's his limit.
Honestly, because of how the time travel works, I'm not even sure if it really has a limit, since Saitama
converted his body into antimatter and it didn't really seem like he had to "maintain" that state, just converting his body back into normal matter to do the Zero Punch. It seems like it's only initiating and ending the time travel that requires any actual "effort".
Yhwach literally snapped Ichigo's zanpakuto in half the moment he was using Bankai and his merged hollow form together, sure maybe Yhwach would brag at the start, but I don't see what's stopping Yhwach from picking a future that favors him, especially when Saitama starts growing so much in power that I don't see why Yhwach would mess around and find out.
I'd like to take this time to point out that the 8.5x AP difference is primarily because of Blut Arterie, an ability Yhwach has to activate manually. Without it, he's actually
weaker than Saitama. He also
can't use both it and Blut Vene simultaneously. Saitama's growth is
so fast that by the time Yhwach notices he's getting stronger, he'll already be so far ahead that Yhwach will be forced to either use Blut Arterie to compete and risk getting oneshot (which won't kill him permanently because of Immo Type 4, but will buy Saitama time to get even stronger) or use Blut Vene and let Saitama continue to grow in strength. He'd also be so much faster than Yhwach by this point that I don't see what he could even really do.
Set traps maybe? But he's going into this with no prep time and Saitama's already almost 5 times faster than him at the start.
But just like you with Saitama, I'm not saying Yhwach
can't win, it's certainly reasonable that he could, but in order to do so, he'd have to very quickly notice Saitama's growth and immediately body him with Blut Arterie before he pulls away from him and Zero Punches his ass.
Also why are we acting like Saitama is on some Medaka Kurokami type stuff with his power mimicry, he's not copying the Almighty when he has yet to prove he can copy Fate hax and stuff like that.
He actually kinda
is on that shit lmao. Garou
states that Saitama can "copy his techniques and surpass them even further" (sound similar to a
certain someone?) which should include his portals, and Saitama demonstrates this by copying what Garou calls his "most powerful ultimate technique of [his] fist" which not even he was able to actually use.
I'm not saying he can copy the Almighty fully (considering how absolutely busted it is compared to anything Saitama has copied before, I'd doubt it), but even a lesser version of it would still be a problem for Yhwach. But if he
can copy it fully... Yhwach's completely cooked. He just has to watch as his only wincon vanishes before his very eyes, while Saitama still has the Zero Punch.