I'd say it's going to depend on your storage conditions. If you have it in a place that is very high in humidity, and where the temperature fluctuates up and down dramatically, then 6 months is probably a good estimate. On the other hand, if you store it in a cool, dry place with very little temperature fluctuation then it could last several years.
Everyone has different ideas about trophy pictures. Looks around and you will see "long armers," "set behinders," "big cheesers," and "blood covered brutes." I have a few tips to keep your trophy pictures from being laughers or worse yet just plain gross.
#1 Clean it up. Try to avoid bullet holes, gut pile, mass amounts of blood, and the tongue sticking out. You don't want your big buck to look like a angry 1st grader with his tongue out or a MMA fighter during the 3 round.
#2 Long arming...