it shouldn't take that long, but like everything practice makes perfect so don't panic. When I file off gel I use a new file (about 120 grit) being really careful not to nick the clients skin as they are vicious when they are new! I take off the bulk of the gel with this and then when nearer the skin switch to a 240 grit so that I would avoid any overbuffing on the natural nail. It is the most boring job in the world, and tiring too due to the extra concentration of not over filing, bit it takes me about 30 mins to file off a full set of LCN gel.
It is correct that you should leave a thin layer on whether applying a new set or not ...... however I am a serial picker lol and even manage to pick this thin layer off when I go back to my own nails, so I do file all the gel off in these circumstances as in my opinion the picking causes far more damage than the buffing. I've done enough file offs now to know which is gel and which is nail so there really is virtually no chance of damage occurring, and I can then retail the Nailtiques After Artificial Treatment, which wouldn't work if there was gel still on the nail.