Are there models of single neurons on slow timescales?
The search for a biologically realistic neural network is never ending. As Sydney mentioned there are many newer models of neurons that take into account activity over larger time-scales, such as the Adaptive Leaky-Intergrate-Fire neuron. The bleeding edge of this search is the Blue Brain Project, which is trying to create the most biologically detailed model of the brain possible. The project should be a good resource if you're interested in the more advanced models.

Since you're interested in Artificial Neural Networks (which implies you're also interested in computation power), I would like to point out that the Neural Engineering Framework allows you to use as detailed of a model of neuron as desired on a realistic time-scale, while still allowing you to compute and learn arbitrary functions as a typical ANN allows.