If I eat more than trace amounts of Gluten (ie the may contain traces, which I can get away with) it takes me about 10 days to feel it is out of my system... but I seriously think it took a good 3-4 months before I really felt it had made a difference.
I'm now 2years into "avoid, but not 100% eliminate" Gluten & last night I was thinking that 2 years ago I was on antihistamines all the time & I couldn't even find the packet, for DH who has hayfever, as it is that long since I've needed antihistamines. That said I didn't remove Gluten for Gut issues, I removed it for sinusitis issues, although must say as a side effect, I do know I have had stomach pains after wheat based foods when I do go "yum caramel slice, I'll have some"