We always had 2 x 30 minute sets at DDA, and it worked really well. Plus there was the added bonus that everyone got a decent time (which, in a night that ends at 1 or 2am isn't possible if everyone does 1 x 60 minute set)
I personally don't see what the problem is - 30, 60, 90 minutes? If you struggle to do it then perhaps it isn't for you