Two months' rent as a refundable deposit is the norm across the areas we work in. Some owners ask for three, and a few ask for six.
When more is reasonable
Fully furnished flats with appliances, or a tenant with no local references. In those cases three months is defensible.
When it isn't
An unfurnished flat where the owner simply wants working capital. That is not your problem to fund, and it is negotiable more often than people think.