I just finished an adventure with 2 battle hardened generals and the grim reaper. All 3 of them are taking 30 minutes to return. I also get this randomly when transferring between my island and adventures. Sometimes they take 15, sometimes they take 30, sometimes they arrive instantly. The fluctuation in this is very annoying.
I just had my general take 46 minutes. I was doing pirates and had timed it so that I would be able to bring troops back, but with the bug I ran out of time and didn't get to finish the adventure.
I have been having the same issue also. It seems though that the Travel Times balance out when you look at the to/from times. My Regular General to an Adventure took 45 minutes to get there but only 15 minutes to return to my island. My Grim Reaper took 23 minutes to get to an Adventure but only 7 minutes to return. A little annoying, but it looks like the total round trip time is correct.
I sent a army to do one camp and I was done. It is taking at least 40 minutes to send the reaper. And I could care less about how long it takes for him to get back.