When I did my belt 4 yrs ago, the belt that was originally on it was physically shot, it looked old, worn and brittle. I did the belt at that time, then changed it after two years, then just did it again. Everytime I've changed it in the two year interval I feel it could go longer. Its a gamble I know, but I think garage keeping the car (in a heated garage), and not driving it as a regular vehicle much would lengthen the belt change intervals.
I went crazy this past winter when I did the belt, and used that guitar tuning software, however I can do it without it im pretty confident as I did it the first time around.
I think its best to spread the project over the off season (at least in the northeast) when you arent rushed, can take your time, order parts you may need, etc. (belt tension bearing, other belts, etc.) Rather than do it in the midst of when you might really really want to drive the car, like when its nice weather. I never thought I could have that kinda patience on a project, but this car has sorta taught me that you have to wait for proper parts sometimes.