Model for predicting duration based on categorical data
So, from what I understand of the question, you are asking how to model the duration of a job, given the input (which includes City and ClientType).
In this case you can use something like a feedforward neural network to model this problem. You might find, that using this methods, the level of prediction error will be lower than that produced by your model, which could act as a baseline to see if these models do work better for your problem.
When representing each of the categorical variables, we use something called one-hot encoding. Then we concatenate these categorical variables into one n-dimensional input vector, which represents all the features for one example.