# What is an acceptable proportion of events in logistic regression?

In market research I'm building a logistic regression model to estimate the likelihood that clients may change banks. The proportion of events is roughly 10% in my sample. From university I remember that a proportion of events that is too small introduces bias into the estimate. Or is it the standard error that gets biased? (Question 1)

As a rule of thumb, what is an appropriate proportion of events? (Question 2) Please give a straight and rough answer, rather than a technical explanation supported by various references.