Hi. I'm online and happy to answer your question today.
That is a good question. Without knowing the specifics of your case, what exactly was done during the first surgery and why it failed I can't give you a meaningful number.
That being said, it sounds like you have a pars plana vitrectomy, gas/fluid exchange and endolaser. I would expect that the next step is a scleral buckle, more laser, possibly cryotherapy and, maybe, even silicone oil placement.
With these techniques most retina detachments can be anatomically reattached, all other things being equal.
As you stated it is possible to use a longer acting gas to form the bubble, thereby giving the retinal tear longer to seal. If he used SF6 he might be considering using C3F8, which lasts about 3 weeks or so, compared to 4 days or so of SF6.
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