Actually, ich in the free-form dies without a host in approx. 55 hour at 68 degrees F. Or so I read. We all know that ich's lifespan can be sped up through heat, right? And since we keep most of our tank's temps at around the 72-78 range (I keep mine at 74), I tested this out. My boraras urophthalmoides had ich and I kept them in my tiger shrimp/CPO tank. So, I took my fish out into a QT, medicated them and dumped them back in. The original tank was fishless for 4 nights and 3 days. My boraras did not get ich again when I put them back into the tank. If ich truly did take 2 weeks to actually eradicate in a fishless tank, then they would have gotten it again. I think 2 weeks is a bit overkill on the "just to be cautious" but of course, to each his own.
Now, to solve your problem. You can add salt and pump up the heat for the ich treatment even without moving your snails out. I have done this many times before that had spixi snails in them. I never ran into any problems, whatsoever. However, this salt process + heat takes a long time. Usually around 2 weeks for me with this method.
Or....
If you can still count the number of guppies and fish them all out, do so and put them in a QT. Medicate them through actual fish medicine. This clears ich up much faster. It takes 3-4 days to medicate them and be 100% sure that ich is finally gone. After that, simply dump them back into the tank. There shouldn't be any surviving ich left in the main tank.
