My and my 2 boys (11,13) did it at the end of July.
My advice
1. Take a lifestraw. Plenty of opportunities for water. We had 2 water bladders. We would refill both at every crossing and rarely did we use any of the second one.
2. Printed maps in color and pay attention to your milage, count water crossings, etc. The trail isn't hard to follow if you pay attention (we should've been more attentive)
3. Counter Clockwise and get the hills out of the way early on fresh legs
4. Lots of water crossings. Take water shoes to keep the socks, boots dry. There was a section we just hiked in our water shoes because there were so many crossings.
5. MOST IMPORTANT. Check the Little Missouri Gage at Langley and follow the rule of 4' or lower. If it is higher, do not hike and watch the weather for impending rain. The river rises fast and there were multiple fatalities at a campground because of the rising water in the middle of the night. I think there are some options. Explore those online prior to trip.
6. The forest service roads are very slow going and go by several names and some are poorly marked. Cell service is spotty, at best. Take road maps and give yourself plenty of time to find your trailhead.
Our story:
Our goal was accomplishing something rather than seeing the sights and fishing, etc. We wanted to test ourselves. Mission accomplished.
We arrived later than expected on Thursday night and so we slept in the Suburban and got out Friday 6am at Athens-Big Fork North TH and went counter clock wise once we hit the loop. Pretty easy to follow but there are a spur on Brush Heap that we accidentally went down that cost us some time. With the added detours, we did about 14-15 miles the first day. We camp before we hit the big river crossing at Viles branch/Little Missouri junction. So we did the part with the most elevation the first day. It was cold that night (End of JULY and cold, caught me off guard, plan accordingly) and we didn't get much sleep so we were up and out at first light.
Day 2 started great and the river was low and slow for crossing. The river area had some crazy flooding the month or so before we arrived and we couldn't find the trail after crossing. We spent probably almost an hour walking 300 yards in the direction of the trail with 50-100 feet between us. We were spread out looking for markers with debris (trees, trailer house steps, fridges, etc) at least 6 ft tall in spots. The trail markers were either down or washed away. I didn't want to go backwards to the car and abandon the hike. So we used the handheld GPS and bushwhacked up and down some elevation and thick stuff to the road. We were able to use the road and followed it to the next trail head (another detour).
At this point, one of my 11 year old was pretty mentally beat. He thought it was physical exhaustion but talked through the how/why/plan etc. He & I had just done 13 miles in 1 day at Big Bend in April. I had to remind him of the struggles he overcame on that trip. We sat and talked and re-grouped. I asked if they wanted to slow down and spend another night on the trail or finish. We emptied his pack into mine (new pack weight=40-45 #s) and his brother's (new pack weight (25#'s) and he became the front man for the 3 of us. His job was to keep the pace. We finished at 9pm that night. 15 hours that day on our feet. The handheld GPS said we did about 18 miles the second day with the detour and road diversion. I thought we had done a good job of hydrating and taking care of our bodies. When we got to the suburban I drank a normal bottle of water and cleaned up and took off. About 10 minutes on the way home, my head was killing me and I felt nausea. My forehead got tight and I felt like things were not good, my body was quivering. We got to town and stopped at Sonic and I was hesitant to eat because of feeling so bad but I ate something. In about 30 minutes, it started to get better. After consulting some RN's and LVN's I work with, it sounds like I had Hyponatremia (Lack of sodium) which can be really scary. I am going to manage my intake differently from now on.
I was super proud of my boys and the trip was a great time of sharing & bonding, overcoming adversity and digging deeper than usual. 33 miles-ish in 2 hiking days. I am so glad we did it and glad those 2 young men (plus their dad) know adversity and how to dig deep & use what God gave them to get through. It was the kind of trip we will all 3 remember forever.
Good luck to you. If you have any questions, I am happy to tell you what little I know.