How far out to plan an ultra

I think it's dependent on what your current base/fitness level is. Obviously someone that is in Ironman shape will be further along than someone coming off the couch. Also think it depends on your goal in the event (race or survive).

Without knowing anything though, timewise for the event, I would say a 50 miler could be comparable to the time you would put in training for an Ironman. However, even then it's hard to say. To me, straight running is so much more difficult than being able to break up the time with swimming and biking.

Given this is my first year to try one I'm taking the whole year to build up to a 50K in the fall.  Once you have that kind of base Im sure you could do them more often depending How long they were and what kind of shap you were in after the race. So I say at least a year for your first race so you can build up slow and remain injury free. 

As others have said it really depends on what type of base you have. If you have none, then more than a year would be required, but if you have done an ironman, I would think that 6 months would be plenty of time. Ultra marathons are half fitness and half mental.