Some of this may depend on the variety of culture you are using. I am working with a culture that seems to do the best at 85F-90F and about 30 hours of incubation. For some reason, if I push it past 30 hours, it starts to turn soft. This one also seems to like split peas and soybeans more than any other medium I've tried.
I used to work with a culture from GEM cultures that performed well, all the way to about 46 hours, making a very firm, good tasting cake. Unfortunately, they don't sell their tempeh culture anymore. I'm always interested in trying other cultures. Where did you get yours from. Mine is from http://a.co/8AcwUYA. I'm looking for one that will perform better than the one I'm currently using.