If the problem is gallbladder, then yes, I might want to start with a low-fat diet. But the gall-bladder is cleared out only when there is fat in the diet - otherwise it stagnates and can create gallstones. Dr Natasha McBride (the GAPS diet) talks about how to go about this by slowly adding in fats. Her diet heals and seals the gut wall and allows the digestive system to rebuild. I'd highly recommend reading her book and the online resources (it's basically a sub-set of WAPF diet for gut healing). Also look into liver/gallbladder flushes - there is a lot of information online about these - they are pretty intense though - not something to take on lightly.
In terms of how long to try things out - I think it depends on what you are trying out. If you are trying a whole foods diet to heal leaky gut, it will probably never work (in my opinion), but if you are trying out GAPS to heal a leaky gut, then I'm guessing almost everyone would see improvements in 6 months (some in days, others in weeks, but everyone by 6 months). We have been on GAPS for two years now, and can finally eat cheese and raw milk again (we have been dairy free for 6 years) because we have healed our guts through a good diet, and time. Also, as I mentioned on another thread, it took close to a year on GAPS for my sons warts to just disappear.