hello John:
In my opinion a few weeks between finishing the timber faces should not create too much of a problem providing that you take some remedial action. What might happen is that the sun will have a tendency to dry out the upper surface and this will in turn curve the timber concavely on the upper side, especially a timber that is already squared. If the timber is round and the bark is yet on it, this will be sufficient to protect the timber from this twisting. As you hew though and you come down to the final side you may find that when you snap the line from end to end on a timber this length some warpage may show up. What I suggest is that on the final side you stretch the line full length and snap only from the centre to the ends keeping the centre width to the 8"+ (in this case) Note: I usually hew slightly over the finished measurement between 1\4 and 1\2", this allows for shrinkage and error in the hand work. Wood always works better slightly cold, but the main feature is to hew it when the wood is green. Hosing it down will keep it in the proper state especially if the weather is extremely hot, and you are only working intermittenty. Don't forget to protect the finished timber from direct suns rays I usually cover the upper surface with 1" rough boards, this let the timber continue to dry slowly underneath.
Hope this helps
The Nothern Hewer